Reptiles of North America: A Guide to Field Identification (Golden Field Guide from St. Martin's Press)
"Reptiles of North America: A Guide to Field Identification" by Hobart Smith is what I have read for years. A lifelong novice, but curious follower of herpetology, I have used this field guide to identify geckos, skinks, snakes, turtles, terrapins, tortoises and more.
Although I live in the northern Midwest, we still have a few things crawling around. I first had a copy of a much earlier edition when I was a child. We lived on the then-edge of the suburbs, with woods and creeks close by, and this became a useful book, along with a guide for amphibians, spiders and butterflies.
Each animal is described by family, subfamily, species and subspecies, with color-coded maps telling the reader where it can be found. A brief overview of each family provides a few lifestyle facts. Color drawings for every species accompany the description, with details like "2 1/4 in. (58 mm) snout to vent" (Texas Banded Gecko).
A section at the end (20 pp) explains general reptile biology, distribution, scaling, and reproduction.
If the random snake slithers through your lawn, "Reptiles of North America: A Guide to Field Identification" is the book to help you sort out just what kind it is.
I fully recommend this book.