They're not books, but they're VERY good source material, and handy for field use. Check out the card decks for Eastern and Western U.S. plant I.D. at Brigade Quartermaster... they've also got a deck that's not divided east to west, it's good too, but doesn't cover as many plants. I've got them all, along with quite a few books, and the card decks are what I take on foraging trips.

Troy