There is precious little in English on the difficult path to independence of Latvia in 1919, but at least there is more than is available for Estonia and Lithuania at the same time.
When reading material in English on these campaigns it pays to be very suspicious of any troop numbers quoted – almost all of them are wrong, often dramatically exaggerating the strength of the Freikorps.
German Freecorps in the Baltic, 1918-1919 by Charles L Sullivan in Journal of Baltic Studies;
The German Role in the Baltic Campaign: Spring 1919 by Charles L Sullivan in Baltic Review; and
"The 1919 German Campaign on the Baltic: The Final Phase" by Charles L Sullivan from The Baltic States in Peace and War: 1917-1945.
In my view these three articles are the best introductory discussion of the Freikorps in Latvia.
L'Aventure Allemande en Lettonie by Lieutenant General du Parquet
Du Parquet was an eyewitness to many of the events as part of the French Military Mission to Latvia. I have done a rough translation into English of most of this book, avoiding the more tedious political stuff, for those that are interested:
The Outlaws by Ernst von Salomon
Quite a famous book, originally in German as Die Geächteten. While a novel, he was an eyewitness, so it is useful for the view it gives of the motivation of the German rank and file.
Histoire de l'armée allemande : 1918-1946: vol 2, La Discorde (1919-1925) by Benoist-Méchin
This has been translated into English, but is hard to find and anyway is dated and not terribly reliable (he follows du Parquet most of the time anyway).
Cowans War: the Story of British Naval Operations in the Baltic, 1918 - 1920 by Geoffrey Bennett
Recently reprinted as Freeing the Baltic. Covers the naval aspects, so important to the battle of Riga, very well.
Eesti Vabadussőda 1918 – 1920
The Estonian official history of the Independence wars is well worth a look (youll probably have to buy it, but the reprint is cheap enough). Really good maps, nice pictures and even some contemporary aerial photographs of the battlefields of the Cesis campaign.
Latvijas Brīvības cīņas 1918 – 1920
The Latvian encyclopaedia of the war is also cheap and has nice maps and photographs too.
Der Feldzug im Baltikum bis zur zweiten Einnahme von Riga (Januar bis Mai 1919)
Die Kämpfe im Baltikum nach der zweiten Einnahme von Riga (Juni bis Dezember 1919)
The two relevant volumes from the Nazi official history of the Freikorps wars (Darstellungen aus den Nachkriegskämpfen deutscher Truppen und Freikorps). I have translated into English the orders of battle in them. They also have some maps, but they are only detailed for the German victories, most of which were so comprehensive as to be boring gaming.
After that we get down to generalist books (which are not worth the bother if you have read the Sullivan articles listed above) and some specialist literature dealing with small aspects, such as memoirs of members of the Allied evacuation commission.
Vanguard of nazism : the Free Corps movement in postwar Germany, 1918-1923 by Robert GL Waite
Hitler's heralds : the story of the Freikorps, 1918-1923 by Nigel H Jones
Histoire d'un fascisme allemand : les corps-francs du Baltikum et la révolution by Dominique Venner
Baltikum : dans le Reich de la défaite, le combat des Corps-francs, 1918-1923 by Dominique Venner
Several books on the Freikorps have chapters on Latvia, such as the ones above, but these are too brief to be particularly illuminating. Generalist books on the Freikorps tend to regard the Baltic very much as a sideshow, and aren't particularly interested in its peculiarities.
The German Freikorps: 1918 – 1923 by Carlos Caballero Jurado, from the Osprey Elite series
Nice, if brief, but barely covers the Baltic. It tends to exaggerate the prowess of the Freikorps, but is perfectly adequate for uniforms.
Latvia in the Wars of the 20th Century by Visvaldis Mangulis
Can be found on-line but is not always terribly accurate, at least with regard to the military aspects, and is very patriotically slanted.
L'Indépendance des pays de la Baltique 1918-1920 by Jean-David Avenel and Pierre Giudicelli
One of the most disappointing books I have ever bought, being largely off-topic and very badly produced.
My Mission in Finland and the Baltic by Rüdiger von der Goltz
The English translation of Meine Sendung im Finland und im Baltikum, by the Freikorps commander. Don't bother. Von der Goltz was writing propaganda to exonerate himself, and there's an enormous amount of outright lying in this book, especially that the campaign was about defeating the Bolsheviks (when it was always about retaining the German army in being). There's little military detail anyway.
Of course there are an enormous number of books on the topic in German, both histories and memoirs. I have collected most of the important ones, but since I can'tread German, there is not a lot I can do with them.
I also used a number of sources which only supply background or have very small snippets of direct information:
The Reichswehr and the German Republic 1919-1926 by Harold J Gordon
The British Navy in the Baltic, 1918-1920 Journal of Baltic Studies 1976 by W A Fletcher
Colonel Emmanuel du Parquets Mission in Latvia 1919-1920 Journal of Baltic Studies 1992 by S Champonnois
Alex, The Life of Field Marshal Earl Alexander of Tunis by Nigel Nicolson
L'Evacuation des Pays Baltiques par les Allemands by General A. Niessel, Paris, 1935
L'Intervention navale alliée en Lettonie , Octobre-Novembre 1919, Revue Historique des Armées, n° 1, 1995 by Ludovic Chevutschi
Avec le General Niessel en Prusse et en Lithuanie, la Derniere Defaite Allemande by Captain Rene Vanlande
L'Invasion Moscovite by Georges Popoff
La Pologne en lutte pour ses frontières : 1918-1920 by Adam Przybylski
La république d'Estonie by Henry de Chambon
The Latvian impact on the Bolshevik Revolution : the first phase: September 1917 to April 1918 by Andrew Ezergailis
If you want a copy of the material which is out of copyright – the du Parquet, Darstellungen aus den Nachkriegskämpfen … and various memoirs in German – then contact me .