Links
Macaulay Library
All About Birds
John Muir Laws
Internet Archive Bird Books
Handbook of Bird Biology (Cornell Lab of Ornithology) 3rd Edition (paid)
Ornithology by Frank B. Gill (free)
I'm currently combing through the IA OpenLibrary to create a large list of mostly field guides and scientific stuff, but feel free to email me about any book relating to birds, and I'll check the the OpenLibrary for it!
Email me at birdxnerd2008@gmail.com!