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Influence - The Psychology of Persuasion by Cialdini.

Don't Make Me Think: A Common Sense Approach to We Usability, by Steve Krug:

The 22 Immutable Laws of Marketing - Violate Them at Your Own Risk!

Information Architecture for the World Wide Web: Designing Large Scale Web Sites, by Paul Rosenfeld and Peter Morville:

Why We Buy: The Science of Shopping, by Paco Underhill