Collins Tree Guide (Collins S) I have 2 copies one at home one at work
The Arboriculturalist′s Companion: A Guide to the Care of Trees Very handy reference tool
Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propogation and Uses love this one
Diseases of Trees and Shrubs brilliant book
Insects that Feed on Trees and Shrubs: Exotic European Travel Writing, 400-1600 (Comstock Book) so good for putting you in the right place