As with most things regarding Polish history, Norman Davies would be the author to turn to when it comes to to english language sources.
White Eagle, Red Star would be a good start (though mind you, he might be considered somewhat pro-polish biased, having a Polish wife). Also Richard Pipes
The unkown Lenin has a lot of good information, especially when in comes to sources.
For non-english language sources, you can try finding Denikin's
Journals (google books doesn't have them though - they are quite rare and only in russian, though I come accross extracts from them being translated into various languages). Not dealing with the Polish-Soviet War per se, they give an interesting view on its prelude (mind you, they are
very biased, as you can expect from the author)
http://books.google.sk/books?id=o3F...hl=pl&sa=X&ei=0GJ0T-yWEoKVOu36xUw&redir_esc=y