1) The Four Steps to the Epiphany: Steve Blank
Why: Blank is the granddaddy of the Lean movement. His early ideas about Customer Development created the foundational principals on which Lean is structured. The first 3 chapters should be required reading for anyone starting a company.
2) The Founder's Dilemmas: Noam Wasserman
Why: there are a lot of hard questions that need to be dealt with in the early stages of founding a company and this book makes you confront a lot of them, especially noteworthy are the sections on how to deal with co-founders.
I could list a dozen more, but those are two standouts for me.
Good stuff for SaaS startups
Customer Lifetime Value Calculator
Good stuff on working with Technical folks