NPC Essentials is a collection of tips, techniques, and aids designed to help game masters inject detailed NPCs into any role-playing campaign. Inside readers will find advice on designing, role-playing, and managing NPCs during the entire lifetime of their campaigns. Also included are NPC archtypes, encounters, charts, and an example NPC centric adventure. Written by Johnn Four of RolePlayingTips.com and Dragon magazine fame.
This book is rare, sadly. It's too bad, because it's full of smart insights about how to develop fun, believable NPCs, how fully to develop them based on their function in the game, how to introduce them most effectively, and tons of other advice. I particularly admire the section describing potential sources of an NPC's power.