I found this site the other day that has thousands of books about birds(and any other subject) available for free. Many you have to read online but some of them have a .pdf file you could download as well. There is some really cool stuff. From obscure scientific studies, to bird guides for most states/regions, to historical ornithological texts, etc. Just about anything imaginable. It is really cool to read some of the ornithological books from the 1800's about the avifauna of my home state. It is a great place to spend some time and learn.
http://openlibrary.org/search?subject_facet=Birds&subject_facet=Accessible+book











