Identifying snakes

What is a good guide for identifying snakes?  Here is southern Illinois we have had a huge amount of rain.  I am finding snakes in all sorts of places.  I dont have a snake phobia, but i have encountered a Copperhead today.  I have also encountered several kinds i am not familiar with.

i usually go for field guides, like audubon or petersons. i've seen a few recently that i hadn't seen here before, too.

I just dug up pictures from the Illinois DNR.  It appears I encountered a Cottonmouth yesterday in my barn.  It is kind of a tricky identification. There appear to be many possible variations in the coloring.  But the Cottonmouth is the only one that comes close.  This is interesting because I am a little north of it's normal range.  Then, with the warming and excessive rains, it is certainly not impossible. Heck, I have spotted a couple of Armadillos in recent years.

Excuse me. I just re- read my first post. That should have read, "I encountered a Copperhead this year."