Field Guide to Trees of North America
This is the best tree book I've ever seen and almost the only one I use. It features over 700 varieties with over 2,000 color photos. It's absolutely chock full of great information, including identification tips, range maps, abundant general info on trees and much more. And it has a waterproof cover!
A truly remarkable deal at 19.95.
"If you want to improve your bonsai, learn more about trees."