If you don't mind black & white line drawings, I think the 1st publishing
of Britton and Brown's "An Illustrated Flora of the Northern United
States and Canada" is out of copyright. The 3 volume set is published by
Dover Books. You could scan those in if I'm right about the copyright.
I've been looking for software keys to plants for years. I think it
would be a worthwhile project. Good luck! --Charlie