Sprucebunny:
George Petrides Field Guide to Trees and Shrubs is the best you'll find. Its sponsored by Audubon. I don't care for Audubon books per se because the color photos don't describe much, but Petrides guide book stands above the rest. A talented artist with detailed sketches and keys to identify trees by leaves, twigs, buds, pith , flowers and fruit will teach you alot more about trees than color pictures with over exposure and poor brightness/contrasts. If you want a companion book to see its uses, habitat, range etc.. get one of the regional Audubon Tree books.
cheers