I was looking through Project Gutenberg, a site that has been
mentioned here I am sure several months back. I came across "Handbook
of the Trees of New England", by
Lorin Low Dame and Henry Brooks.

http://www.gutenberg.org/files/20467...-h/20467-h.htm

It may not have all the wonderful color plates that are in the Audobon
books but is nonetheless a fine (and free) resource.

Project Gutenberg has made available lots and lots of free books as
well as recordings of folks reading books.