In addition to Four Steps to the Epiphany, Crossing the Chasm, and Getting Real, I would add All Marketers are Liars by Seth Godin. Of Seth's other books, Permission Marketing, Free Prize Inside, and The Big Red Fez also made a big impact on me. But All Marketers are Liars is the best starting place I think.

Definitely also read Art of the Start. It's less insightful and more informative, but it's still a good overview of how the startup process works and it's easily readable in a day.