Awangarda Polish Lowland Sheepdogs
Published in 1995, this book is widely accepted as an excellent resource on the history of the breed. 

It is very well written and tells the story of the PON with photos and graphics. 
This book by the late Betty Augustowski, founder of the Amercian breed club is an excellent resource and gives good information on PON ownership.
This book by Gail Forsyth is available on Amazon.  Looks like a good resource for a first-time PON owner (or any PON owner).