From publisher blurb:
A Whole New Frontier of Gunslinging!
Guns may not exactly be a staple of fantasy settings, but that doesn’t mean that they can’t add a wealth of flavor, fun, and excitement to your game. With the gunslinger class, from paizo’s Ultimate Combat, and the rules for firearms also included in that book, guns have officially been introduced to the Pathfinder Roleplaying Game, and have also been re-envisioned for the 3.5 OGL rules in general.
This book aims to expand on that achievement by providing three firearm-themed prestige classes, each of which approaches the gun in a different way. The book includes the gun mage, who mixes guns and magic for surprising and powerful results, the hair-trigger renegade, who specializes in high-capacity firearms and in making several shots in a round, and the sharp shooter, who specializes in careful, well-aimed shots designed to blow past the target’s defenses. Each of these classes has mechanics designed to make them feel unique and set apart from each other, as well as others who use firearms, and each has several exciting new class features which will make playing with guns more fun than it’s ever been.
While most of these classes will benefit from a character taking levels in the gunslinger class, none of them require it, though players will need access to the firearms rules presented in Ultimate Combat.