Blink - Malcolm Gladwell. The power of 'thin slicing' - an extraordinary read.
Don't make me think - Steve Krug. Very useful, particularly in marrying design with usability.
Design of Everyday Things - Donald Norman - Everyone should read this.
Marketing to the Social Web - Larry Weber - A most useful read.
Meatball Sundae - Seth Godin. Another pacey read from Seth - never quite gives you enough 'how to' however, but always gets the mind going.