Influence: Science and Practice
BY ROBERT B. CIALDINI

We tripped over this book in 2001, and it changed the way we looked at design (and sales people). Cialdini offfers a thorough examination of the psychology of compliance (i.e. uncovering which factors cause a person to say ÒyesÓ to another's request). This classic text in the field of marketing psychology should be part of every designer's education.
Related: Author's site

The Tipping Point:
How Little Things Can Make a Big Difference
BY MALCOLM GLADWELL

This text overlaps a bit with Cialdini, but with totally different focus. It teaches the designer about how people tick and the way society tocks in response. Tipping Point actually lives up to critical statements like "a fascinating book that makes you see the world in a different way." Also: Great design story about the Columbia Record Club and McCann Erickson.
Related: Author's site

Selling the Invisible:
A Field Guide to Modern Marketing
BY HARRY BECKWITH

Particulary valuable for designers because we are faced with the vagaries of selling our services, which are intangible and invisible. Beckwith offers cogent advice on how to nuture and maintain effective relationships with clients. Quick, easy read.