Dread Brass Shadows (Garrett, P. I. Series #5) by Glen Cook
A treasure trove of sorcery is hidden somewhere in the city of TunFaire, and everyone wants to find it—a prize so rich it makes even a hard-boiled detective like Garrett nervous, especially when his beautiful red-haired girlfriend is attacked because of it. Garrett's thirst for revenge is quenched when two other redheads turn up at his front door—and he finds himself the center of unwelcome attention from every thug and would-be sorcerer in town.
Now, Garrett has to find what everyone is after—the legendary Book of Shadows, made of brass and holding secrets no mortal was ever meant to master—and make sure no one ever has the chance to work its spells on his unsuspecting and unprotected world.