By Robert B. Cialdini
This book is a classic for a reason - I've taken my sweet ass time getting to it, but I am absolutely hooked. Drawing on a combination of scholarly experiments and "in the trenches" experience with sales training, this book points out thenearly automatic triggers we have evolved to be good, cooperative members ofsociety and how "compliance professionals" (salesmen, recruiters, and cons) use these techniques to get their way without you even knowing what they're about. This book is doubly valuable - you learn how to spot unscrupulous applications of the techniques as well as how to use them in your own (hopefully legitimate) endeavors. For lots of interesting extra context, check out Scott Adams's Persuasion Reading List.
Buy it here.