For a general field guide with keys, I would recommend The Audobon Society Field Guide to North American Reptiles and Amphibians. There are other works more specific to the American North West, I'll look up some titles and post them for you.

There is one piece of work that might interest you...The Garter Snakes of Western North America, by VanDenburgh and Slevin. The only problem is...it was published in 1918!!