A book considered the garter snake bible is "The Garter Snake, Evolution and Ecology" by Rossman, Ford and Seigel. It has all the identifiers (supralabials, sublabials, lateral count along with lateral stripe row locations, etc.)
Be sure to get the second addition. Every time I read it I learn something new. Great reference for species I.D.
The Garter Snakes: Evolution and Ecology (Animal Natural History Series): Douglas A. Rossman, Neil B. Ford, Richard A. Seigel: 9780806128207: Amazon.com: Books