Ok, to start things out...
Here is a book that I do not really recommend. BUT, to share yet another resource, here it is:
"Handbook of Avian Anatomy: Nomina Anatomica Avium, 2nd Edition", By Nuttall Ornithological Club, Copyright 1993, ISBN-13: 978-1877973345 ISBN-10: 1877973343 779 pages, Hardbound, (I believe it is now "technically" out of print.), Current listed price on Amazon: $195.00 U.S.D. , (NOT cheap!!!)https://www.amazon.com/Handbook-Avian-Anatomy-Publications-Ornithological/dp/1877973343/ref=sr_1_1?ie=UTF8&qid=1516504243&sr=8-1&keywords=handbook+of+avian+anatomy
Like I said, I do not recommend this one. It is a very tedious read. While it IS illustrated, it is not very helpful for artists.