Collins Tree Guide 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 (Comstock Book) brilliant book
Insects that Feed on Trees and Shrubs (Comstock Book) so good for putting you in the right place