Critical Care is correct,Sunset's Western and Eastern Garden Books break down the states hardiness zones much more specifically than USDA zones do.
Sunset developed the Heat Index Zones.
The USDA zones were developed for farming not for ornamental gardening they are much too general and don't have a Heat Index Zone at all.
Sunset Western Garden Book
Sunset Eastern Garden Book
Are THE best to go by because most ornamental nurserys and growers tag their plants using those zones and not USDA zones...at least here in the western states they do.
The best Encyclopedia of Plants though in my opinion is
The American Horticultural Society A-Z Encyclopedia of Garden Plants /Millinium Limited Edition.
It has over 6000 pics and 15,000 Plant descriptions and cultivation descriptions for each.What kind of sun/shade and soil they need.If you make your living planting ornamentals(plants,tree's,shrubs ect)I strongly recommend you invest in this wonderfull book.It will blow your mind!It also uses the same zones Sunset does.
Now some older books still use the USDA zones including old Sunset books so Keep your eye out if you use an old gardening book.