Understanding the way the human mind works can help a lot if you want to design effective interfaces.
100 Things Every Designer Needs to Know About People
A set of practical principles, backed by scientific research, that every designer will find relevant to their work. A short and easy read, thanks to Susan's work in reducing complicated research down to bite-sized wisdom.
How to Get People to Do Stuff: Master the Art and Science of Persuasion and Motivation
This is not a design book, but persuasion and motivation are very important, especially in commercial settings.