It is about like comparing a dial-up connection to a dsl connection or comparing a Volkswagen to a BMW. Seriously though, I own both books and the "Texas Snakes" is much more comprehensive than the "Field Guide" version. The Feld Gude version is OK to stick in your clove compartment or your back pack, while the "Texas Snakes" is best suited for your bookshelf. I find myself referrng to "Texas Snakes" on a daily basis, while the Field Guide version seldom gets opened.
Texas Snakes

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Gerald Keown
Southwestern Center for Herpetological Research
www.southwesternherp.com