There are a couple of excellent books in English on the Polish-Soviet War of 1920 but effectively nothing about the Polish-Soviet campaigns of 1919, nor very much on the simultaneous war by Poland against Ukraine. I have therefore assembled a few bits and pieces that together give an introduction to these neglected campaigns.
Below are several chapters from Przybylski's history of Poland's wars from its renaissance in 1918 to the end of the Soviet war. Since I can't read Polish, I have had to be content with providing a translation of the French version La Pologne en lutte pour ses frontières : 1918-1920. While not ideal, it is at least a start.
For those not familiar with the issues of the war I have provided an introduction. I also added notes to the chapter texts.
Introduction to Przybylski and the Ukrainian-Polish War
Przybylski Chapter 4 "November 1918 – April 1919"
Przybylski Chapter 5 "April – July 1919"
Przybylski Chapter 6 "General Offensive on the Russian Front"
Sadly however, Przybylski's history is very one-sided, both politically and in his military coverage, so I have also found some articles that give at least part of the Ukrainian version:
The reason why I set up these pages is precisely because there isn't very much to find, but a few books are worth looking for in Western languages:
Ukrainian Armies 1914–55 by P. Abbott and E. Pinak, Osprey Men-at-Arms #412
A useful introduction to both the Galician and Directory Ukrainian armies with, most importantly for wargamers, discussion on the uniforms in English. It is a good book on a neglected topic, spoiled only by the publisher's rigid format not allowing anywhere near enough detail.
Армии Украины 1917–1920 Годов (Armii Ukraini 1917-1920) by Ya. Tinchenko (Moscow, 2002)
More detailed on the various Ukrainian units and their uniforms
Натиональные Армии (Nationalist Armies) by A. Deryabin.
Has a tiny bit on the Ukrainians.
Unlike for 1919, where information is quite good, there is no decent single source for information in English (or French) on the Polish army or uniforms of the time.
Войска Интервентов (Interventionist Forces) by A. Deryabin
Натиональные Армии (Nationalist Armies) by A. Deryabin
A small amount of material on the Poles can be found in these two. English translations are available.
A City fights for freedom : the rising of Lwów in 1918-1919 by Rosa Bailly
Translated from the original French, this is a massively detailed account of the battle of L'viv (Lwow), at least from the Polish side. It is however extremely unreliable concerning the Ukrainians.
Semper Fidelis : Obrona Lwowa
A collection of photographs concerning the fighting around L'viv. Descriptions are in Polish, French and English, but sadly photos are only of the Poles.
Below is some supplementary material that Henryk Szot kindly directed me to:
The Lands of Partitioned Poland, 1795 - 1918, Piotrs. Wandyz, 1974
Volume VII in A History of East Central Europe, Ed. Peter Sugar, Donald Treadgold, University of Washington.
This book presents a fairly neutral view of the Polish-Ukrainian relationships, although more on preparation for the wars that the fighting itself. It provides details on Polish (in Austria, Russia and Prussia) and Ukrainian Armed Forces prior to 1 Nov 1918.
The Ukrainian-Polish Defensive Alliance, 1919–1921, An Aspect of the Ukrainian Revolution by Michael Palij, 1995.
The book is primarily devoted to the politics of the situation, but some information on the battles is included.