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.
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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.