The "Invincible Black Brigade: Polish 10th cavalry brigade 1939" by Jerzy Majka. This is a soft cover, A4 format, 120 page book focusing solely on the Polish 10th motorized cavalry brigade, which is one of the most famous combat units from the September campaign, and easily the most distinguishable in terms of look, organization and their combat track record.
The book may be thin, but it comes with valuable information about the organization of the 10th motorized cavalry brigade - complete with a spreadsheet of formations within the brigade as well as a superb roster that tells you exactly how many soldiers, officers, vehicles, weapons etc each unit had.
Check my blog for more info and a lot of pictures of the content:
http://anatolisgameroom.blogspot.se/2013/10/the-invincible-black-brigade-book-review.html