National Audubon Society Field Guide to North American Wildflowers : Eastern Region
by John W. Thieret, William A. Niering, Nancy C. Olmstead
National Audubon Society Field Guide to North American Wildflowers : Western Region
by Richard Spellenberg
The National Audubon Society Field Guide to North American Trees : Eastern Region
by Elbert Luther Little, Elbert L. Little Jr., Angelo Lomeo (Photographer), Sonja Bullaty (Photographer)
National Audubon Society Field Guide to North American Trees : Western Region
by Elbert L., Jr. Little, Angelo Lomeo (Photographer)
National Audubon Society Field Guide to North American Mushrooms
by Gary A. Lincoff, Gary H. Lincoff
Web Sites:
Online Image Databases 
Plants
Tree of Life Web Project, provides scientific name for species
Google Image Search, searchable database of images