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If you're okay with going back to 3.5, the "Forgotten Realms: Serpent Kingdoms" book has a lot of information on everything serpent-y.
Of course, Serpentfolk themselves aren't featured in that book, and the setting isn't Golarion, so it might not suit your needs (Yuan-Ti served a similar role, though).
James Jacobs
Creative Director
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I first introduced the serpentfolk to Golarion in "Into the Darklands." They have a fair amount of information about them there. They also play a significant role in the last few Pathfinder Journals of Eando's journey (particularly in Pathfinders #15–#17, if I recall correctly).
But yeah... the BEST place to go is the Serpent's Skull Adventure Path. The ecology, as mentioned above, is in the first volume, but all six volumes of this Adventure Path are loaded with serpentfolk lore. As one would expect from an entire campaign focused on serpentfolk.
In any event, our serpentfolk and D&D's yuan-ti are, as far as I can tell, both inspired by the same source—various Robert E. Howard stories (particularly some of the Kull stories) and some of Lovecraft and Clark Ashton Smith's writings.