a very good book for field herping is 'snakes of north america' by Alan Tennant, it comes in 'eastern and central' and 'western editions'. It has a great deal of info on every native snake and seperate descriptions for each subspecies.
The catigories for each species or subspecies listing are
Venomous / Non-venomous - this really tells you about defencive behavior more than venom.
Abundance - address conservation issues
colouring / scale form
these are very informive and are complete for each subspecies, though some subspecies do say 'see such and such'
there are 400 pages of species discription and 122 pages of photos (in the east / central edition) total 605 pages, covers 196 species / subspecies. Retails for $29.95 USD on the back cover, I bought it in Canadian and don't remember what I paid.
A very good book I think and incredibly helpful for IDing subspecies.