[.ca] Manual of Ornithology: Avian Structure and Function (ISBN 0300076193)

Excellent drawings, clear text.:
If you have only one textbook about birds on your bookshelf, choose this one. The text is easy to understand, and the illustrations are remarkable. It is now available in a less expensive paperback version, also available at Amazon.com.

good source for avian physiology:
This book is excellent for ornithology students and backyard birders alike. It has many detailed illustrations of the skeletal, muscular, and other internal systems of birds in general. The text gives general information on each system and then more detail on specific parts. I especially like the sections at the end on binocular use and study skin preparation