The Fundamental Four

Here are the four most important books recommended by Timothy Ferriss from The 4 Hour Work Week:

The Magic of Thinking Big by David Schwartz
How to Make Millions with Your Ideas: An Entrepreneur's Guide by Dan S. Kennedy
The E-Myth Revisited: Why Most Small Businesses Don't Work and What to Do About It by Michael E. Gerber
Vagabonding: An Uncommon Guide to the Art of Long-Term World Travel by Rolf Potts

One more worth mentioning is:
Walden by Henry D. Thoreau