If you go to Google Book Search and enter `diseases of poultry' you'll get quite a bit from the GAM comprehensive texts (when you reach the page view limit just delete your temp. files and go back for more).
Downloadable (free but for the time it takes to download) texts that I've found useful:
U.S. Fish and Wildlife service manual on field procedures for avians (lots of good info on just about everything, e.g., toxins, plastic eating shorebirds, etc. Not chook specific, but diseases are thoroughly covered, and gives one pause regarding wild birds and chook biosecurity.