1. The Slayer's Guide to Gnolls, Mongoose Publishing 2001 [MGP0002] -- contains everything from Gnoll physiology to habitat, society, scenario hooks, and more. This is a d20 product released under OGL 1.0, so it's intended for D&D 3.X, but Pathfinder is a d20 game also.
If you're into antiques, you can check ebay and similar places for:
1. The Slayer's Guide to Gnolls, Mongoose Publishing 2001 [MGP0002] -- contains everything from Gnoll physiology to habitat, society, scenario hooks, and more. This is a d20 product released under OGL 1.0, so it's intended for D&D 3.X, but Pathfinder is a d20 game also.
For what it's worth, you can still buy this in PDF.
A Brief History of Gnolls (available from DriveThruRPG) has both the 1912 short story by Lord Dunsany that created the Gnolls, and the 1951 short story by Margaret St. Clair based on that story that Gary Gygax based his gnolls on. If you want to go back to the start, that's a good place to go.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.