In the past I have added items to the Sources page instead of making a separate post, but at this point the Sources page is very long so I thought I would make a separate post for this year's very large update. Below are most of the sources for the rest of the 2019 episodes, although a few more will probably be added along the way.

India

Indians at home, Mesopotamia and France, 1914–1918: towards an intimate history by Santanu Das
India and World War 1 Edited by DeWitt C. Ellinwood and S.D. Pradhan
The Indian Army on the Western Front, 1914–1915: A Portrait of Collaboration by George Morton Jack
An Imperial Home-Front: Punjab and the First World War by Tan Tai-Yong
India and World War 1 Edited by DeWitt C. Ellinwood and S.D. Pradhan
Europe Through Indian Eyes: Indian Soldiers Encounter England and France, 1914-1918 by Oxford University Press
The Sepoy and the Raj: The Indian Army 1860-1940 by David Omissi
Collaboration and British Military Recruitment: Fresh perspectives from colonial Punjab, 1914–1918 by Tahir Mahmood
Army of Empire: The Untold Story of the Indian Army in World War 1 by George Morton-Jack
The influence of racial attitudes on British policy towards India during the First World War by Gregory Martin
Honour and Fidelity: India's Military Contribution to the Great War 1914-1918 by Captain Amarinder Singh

Arms Control and the Anglo-German Naval Race before World War I: Lessons for Today? by John H. Maurer
A Fleet in Begin: The Rise and Fall of Italian Sea Power, 1861-1943 by Brian R. Sullivan
Admirals versus Generals: The WAr Airms of the Imperial German Navy, 1914-1918 by Holger H. Herwig
From the Dreadnought to Scapa Flow: The Royal Navy in the Fisher Era, 1904-1919
The Rules of the Game: Jutland and British Naval Command by Andrew Gordon and Paul Wilderson
Winning Cheaper Laurels: Borkum/Baltic, the Dardanelles, and Churchill's Search for a Naval Offensive in the First World War, 1914-1915 by Graham Clews
Detente and Deterrence: Anglo-German Relations, 1911-1914 by Sean M. Lynn-Jones
'The Spirit of the Army' at Sea: The Prussian-German Naval Officer Corps, 1847-1897 by Lawrence Sondhaus
The German Reaction to the Dreadnought Revolution by Holger H. Herwig
The German School of Naval Thought and the Origins of the Tirpitz Plan 1875-1900 by Rolf Hobson
Imperial Cable Communications and Strategy, 1870-1914 by P.M. Kennedy
The Titan Refreshed: Imperial Overstretch and the British Navy before the First World War by Phillips Payson O'Brien
Krupp and the Imperial German Navy, 1898-1914: A Reassessment by Michael Epkenhans
Left-Wing Opposition to Naval Armaments in Britain Before 1914 by Howard Weinroth
The Unknown Effort: Theodore Roosevelt's Battleship Plan and International Arms Limitation Talks, 1906-1907 by Frederick C. Leiner
Strategy, Tactics, and Turf Wars: Tirpitz and the Oberkommando der Marine, 1892-1895 by Patrick J. Kelly
Tirpitz: And the Imperial German Navy by Patrick J. Kelly
Dreadnought: Britain, Germany, and the Coming of the Great War by Robert K. Massie
The Failure of British Espionage against Germany, 1907-1914 by Nicholas P. Hiley
The Complexity of Strategy: "Jackie" Fisher and the Trouble with Submarines by Christopher Martin
Sir John Fisher and the Dreadnought: The Sources of Naval Mythology by Jon Tetsuro Sumida
The Force of Circumstance: Graf Spee's Options for the East Asian Cruiser Squadron in 1914 by Peter Overlack
The Anglo-German Naval Rivalry and Informal Arms Control, 1912-1914 by John H. Maurer
Nation, Empire and Navy: Identity Politics in the United Kingdom 1887-1914 by Jan Ruger
Strategy and War Planning in the British Navy, 1887-1918 by Shawn T. Grimes
Steaming in the Dark? Rules, Rivals, and the British Navy, 1860-1913 by Richard J. Stoll
'Riches beyond the Dreams of Avarice'?: Commercial Returns on British Warship Construction, 1889-1914 by A.J. Arnold
Strategic Command and Control for Maneuver Warfare: Creation of the Royal Navy's "War Room" System, 1905-1915 by Nicholas A. Lambert
Memories and Records Volume Two by Admiral of the Fleet Lord Fisher
Germany's High Sea Fleet in the World War by Admiral Reinhard Scheer
"Luxury" Fleet: The Imperial German Navy 1888-1918 by Holger H. Herwig
The Grand Fleet 1914-1916: Its Creation, Development and Work by Admiral Viscount Jellicoe of Scapa
The Great Naval Race: Anglo-German naval rivalry 1900-1914 by Peter Padfield

J.R.R. Tolkien

Tolkien and the Great War: The Threshold of Middle-earth by John Garth
Frodo's Batman by Mark T. Hooker
“Tricksy Lights”: Literary and Folkloric Elements in Tolkien’s Passage of the Dead Marshes by Margaret Sinex
J.R.R. Tolkien: Author of the Century by Tom Shippey
J.R.R. Tolkien: A Biography by Humphrey Carpenter
The Letters of J.R.R. Tolkien Edited by Humphrey Carpenter

Russian Civil War

A People's Tragedy The Russian Revolution 1891-1924 by Orlando Figes
The Anglo-Soviet Trade Agreement, March 1921 by M.V. Glenny
Behind the Front Lines of the Civil War: Political Parties and Social Movments in Russia, 1918-1922 by Vladimir N. Brovkin
Civil War in Europe, 1905-1945 by Stanley G. Payne
Civil War in South Russia, 1918 by Peter Kenez
Communism and the East: The Baku Congress, 1920 by Stephen White
The Eastern Front 1914-1920: From Tannenberg to the Russo-Polish War by Michael S. Neiberg & David Jordan
Episodes from the Early Cold War: Franco-Soviet Relations, 1917-1927 by Michael Jabara Carley
Harmonicas for Lenin? The Development of German Economic Policy Toward Soviet Russia December 1918 - June 1919 by Robert Himmer
Red Advance White Defeat Civil War in South Russia 1919-1921 by Peter Kenez
Arkhangel'sk, 1918: Regionalism and Populism in the Russian Civil War by Yanni Kotsonis
Russia and Europe: Diplomacy, Revolution, and Economic Development in the 1920s by Teddy J. Uldricks
Soviet Policy Toward Germany during the Russo-Polish War, 1920 by Robert Himmer
The Russian Revolution 1917-1921 by William Henry Chamberlin
The Problem with Generals: Military Observers and the Origins of the Interventionin Russia and Persia, 1917-1918 by Brock Millman
The Political Significance of German-Soviet Trade Negotiations, 1922-5 by R.P Morgan
The "Russian" Civil War, 1916-1926: Ten Years That Shook the World by Jonathan D. Smele
The Transition from War Communism to the New Economic Policy: An Analysis of Stalin's Views by Robert Himmer
Why Did the Communists Win or Lose? A Comparative Analysis of the Revolutionary Civil Wars in Russia, Finland, Spain, and China by Pavel Osinsky and Jari Eloranta
The Russian Civil War by Evan Mawdsley

Polish-Soviet War

The Entente and the Polish Question 1914-1916 by Alfred Erich Senn
Soviet-Polish Relations, 1919-1921 by Kirsteen Davina Croll
The Polish-Soviet Campaign of 1920 by Marjan Kukiel (1929)
Polish-Czechoslovak Relations, 1918-1922 by Zygmunt J. Gasiorowski
Poland's Policy towards Soviet Russia, 1921-1922 by Zygmunt J. Gąsiorowski
The Military Thought of Władysław Sikorsk by Robert M. Ponichtera
The Politics of Anti-Bolshevism: The French Government and the Russo-Polish War, December 1919 to May 1920 by Michael Jabara Carley
Hands off Russia: British Labour and the Russo-Polish War, 1920 by L.J. Macfarlane
Britain and the Russo-Polish Frontier, 1919-1921 by H.J. Elcock
Lord D'Abernon, the Anglo-French Mission, and the Battle of Warsaw, 1920 by F. Russell Bryant
Warsaw 1920: Lenin's Failed Conquest of Europe by Adam Zamoyski
White Eagle, Red Star: The Polish-Soviet War 1919-20 by Norman Davies
Anti-Bolshevism in French Foreign Policy: The Crisis in Poland in 1920 by Michael Jabara Carley

Finnish Civil War

The Splintered Empires: The Eastern Front 1917–21 by Prit Buttar
From Grand Duchy to a Modern State: A Political History of Finland since 1809 by Osmo Jussila, Seppo Hentila, and Jukka Nevakivi
State and Revolution in Finland by Risto Alapuro
Civil Wars, Revolutions and Counterrevolutions in Finland, Spain, and Greece (1918-1949): A Comparative Analysis by Julian Casanova
The Finnish Army, 1881-1901: A National Force in a Russian Context by J.E.O. Screen
Finnish Military Politics between the Two World Wars by William J. Stover
The Finnish Social Democratic Party and the Bolshevik by David Kirby (1976)
The German Expedition to Finland, 1918 by A. Harding Ganz
Polish-Finnish cooperation in border-state policy, 1919–1922 by Kalervo Hovi
The Problem of Generations in Finnish Communism by Marvin Rintala
Russia and the Origins of the Finnish Civil War of 1918 by C. Jay Smith Jr.
Social Conditions for Political Violence: Red and White Terror in the Finnish Civil War of 1918 by Sirkka Arosalo
The Finnish Civil War 1918: History, Memory, and Legacy Edited by Tuomas Tepora and Aapo Roselius

Baltic States

The Splintered Empires: The Eastern Front 1917–21 by Prit Buttar
The Baltic States The Years of Independence: Estonia, Latvia, Lithuania 1917-1940 by George Von Rauch
The Baltic Germans and German Policy towards Latvia after 1918 by J.W. Hiden
The Baltic States: The National Self-Determination of Estonia, Latvia, and Lithuania Edited by Graham Smith
The Baltic States and Weimar Ostpolitick by John Hiden
Defending National Sovereignty Against two Russias: Estonia in the Russian Civil War, 1918-1920 by Karsten Bruggemann
The Formation of the Lithuanian Foreign Office, 1918-1921 by Alfred Erich Senn
German freecorps in the Baltic, 1918-1919 by Charles L. Sullivan
National Communism and World Revolution: The Political Consequences of German Military Withdrawal from the Baltic Area in 1918-19 by James D. White
The Lithuanian Card in Russian Policy, 1914-1917 by Raimundas Lopata
Soviet Policy Toward the Baltic States 1918-1940 by Albert N. Tarulis (1959)
The Baltic Nations and Europe: Estonia, Latvia, and Lithuania in the Twentieth Century by John Hiden and Patrick Salmon

Greco-Turkish War

Paradise Lost: Smryna 1922: The Destruction of a Christian City in the Islamic World by Giles Milton
Ataturk, Lloyd George and the Megali Idea: Cause and Consequence of the Greek Plan to Seize Constantinople from the Allies, June-August 1922 by Michael M. Finefrock
Great Britain and the Greco-Turkish War, 1912-1922 by Karl G. Larew
The Greco-Turkish War, 1920-1922 by Peter Kincaid Jensen
United States Naval Detachment in Turkish Waters, 1919-1924 by Henry P. Beers (1943)
Ataturk: Rebirth of a Nation by Andrew Mango
Atatürk: The Biography by Kinross, Patrick

Iraq and Egyptian Revolts

The 1920 Revolt in Iraq Reconsidered: The Role of Tribes in National Politics by Amal Vinogradov
'A Tedious and Perilous Controversy': Britain and the Settlement of the Mosul Dispute, 1918-1926 by Peter J. Beck
British Oil Policy in the Middle East 1919-1932 by Colin Davies
The British Press and the Future of Egypt, 1919-1922 by Michael B. Bishku
The Cambridge History of Egypt
Creating Nations, Establishing States: Ethno-Religious Heterogeneity and the British Creation of Iraq in 1919-23 by Guiditta Fontana
Did Britain Use Chemical Weapons in Mandatory Iraq? by R.M. Douglas
Enemy on the Euphrates: The Battle for Iraq, 1914 - 1921 by Ian Rutledge
Competing Narratives: Histories of the Women's Movement in Iraq, 1910-58 by Noga Efrati
Labor Unrest in Egypt, 1906-90 by Donald Quataert
British Oil Policy in the Middle East 1919-1932 by Colin Davies
Ordinary Egyptians: Creating the Modern Nation Through Popular Culture by Ziad Fahmy
The Other 'Awakening' in Iraq: The Women's Movement in the First Half of the Twentieth Century by Noga Efrati
Liminal Loyalties: Ottomanism and Palestinian Responses to the Turkish War of Independence, 1919-22 by Awad Halabi
Patters of the 1920 rising in Iraq: The Rifā'iyya ṭarīqa and Shiism by Thomas Eich
Peasants in Revolt - Egypt 1919 by Ellis Goldberg
Southern Kurdistan under Britain's Mesopotamian Mandate: From Separation to Incorporation, 1920-23 by Saad Eskander
The Egyptian Revolution of 1919: New Directions in the Egyptian Economy by Robert L. Tignor
War and Occupation in Iraq: What Went Right? What Could Go Wrong? by Judith S. Yaphe

Irish Civil War

1922: the Birth of Irish Democracy by Tom Gravin
Green Against Green – The Irish Civil War: A History of the Irish Civil War, 1922–1923 by Michael Hopkinson
A Short History of the Irish Revolution, 1912 to 1927: From the Ulster Crisis to the formation of the Irish Free State by Richard Killeen

Germany After the War

The Anti-Russian Tide in German Socialism, 1918-1920 by William Maehl
The "Black Horror on the Rhine": Race as a Factor in Post-World War I Diplomacy by Keith L. Nelson
Weimar Germany as Seen by an Englishwoman: British Women Writers and the Weimar Republic** by Colin Storer
Lord Hardinge As Ambassador to France, and the Anglo-French Dilemma over Germany and the Near East, 1920-1922 by J. Douglas Goold
Mussolini and the Ruhr Crisis by Sally Marks
Politics of Illusion: Tirpitz and Right-Wing Putschism, 1922-1924 by Raffael Scheck
Rapallo Reexamined: A New Look at Germany's Secret Military Collaboration with Russia in 1922 by Gordon H. Mueller
The Reich Government versus Saxony, 1923: The Decision to Intervene by Donald B. Pryce
The Ruhr Authority and the German Problem by Amos Yoder
Russo-German Military Collaboration During the Weimar Republic by Hans W. Gatzke
The Ruhr Crisis, 1923-1924 by Conan Fischer
The Struggle for Upper Silesia, 1919-1922 by F. Gregory Campbell
Britain and the Ruhr Crisis by E. O'Riordan
Founding Weimar: Violence and the German Revolution of 1918-1919 by Mark Jones
Perspectives on Modern German Economic History
The German Revolution, 1917-1923 by Pierre Broue and Eric Weitz
The Weimar Republic by Eberhard Kolb