Field Guide to Trees of North America
If you want to improve your bonsai, learn more about trees.
This is the one of the very best tree books I've seen and almost the only one I use. It features over 700 varieties with over 2,000 color photos. It's 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 remarkable deal at 19.95