Articles published in scientific journals are generally considered to be the "official source". Field guides interpret and compile data from a variety of sources including these documents. They are general guides, and not meant to be a complete study on the animals they describe. They tend to be imprecise when it comes to fine details, especially ranges, intergrades, and transition zones. A good example would be the range map I just posted, which I believe is from the Conant guide. There is no black rat snake influence in FL as the map shows, nor is there any yellow rat snake influence in TN, AL, or Western GA. The map doesn't even show any intergradation in the Carolinas.

http://www.dlia.org/atbi/species/Animalia/Chordata/Reptilia/Squamata/images/Colubridae/Elaphe_obsoleta/Elaphe_obsoleta_map.jpg

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