I know you only asked for one...but here are my Top 6 books, read in this order for these reasons:
It will help you defend your right to be an entrepreneur.
Here is a Google Reader blog bundle that I subscribe and read to on a daily basis. Let me know what you think!
http://www.google.com/reader/bundle/user%2F16649001462035247274%2Fbundle%2FEntrepreneurship Includes all these blogs here:
There are many books out there that will provide information on running startups, businesses and projects.
To recommend the single most influential book we have to define what is at the core of being an entrepreneur.
I would say that being an entrepreneur is the ability to look at what you have and work to improve it. Inc Magazine recently ran an article on how entrepreneurs think.
Therefore, the most influential book is one that spurs you on to improvement, drives you to work harder, and gives you the confidence and belief that you have the power to take control. It may not even be a business book.
I found that Steven Covey's Seven Habits of Highly Effective People is a good book for thinking about self improvement. You can read about the Seven Habits on Wikipedia.
E-Myth by Michael E Gerber
Its a complete outlook of a startup
I'm actually not that found of Guy Kawasaki. He is a great speaker, funny and all, he gives good advise but his track record is honestly not that impressive in terms of startup.. Try Steve Blanks book, which despite being visually old style (ugly..) it contains great insights that will influence the way you launch your startup and potentially succeed with it.
Book: The four steps to the epiphany
Hackers and Painters - Paul Graham
http://www.paulgraham.com/hackpaint.html An entertaining insight to the mindset of hacking and entrepreneurship.
Emergency - Neil Strauss
http://www.neilstrauss.com/emergency/ It has nothing, and everything, to do with starting a company and being self-sufficient.
However, I don't think there is any "single most important" anything. Read what interests you, and always apply your own critical analysis to the content.
"The Innovators Dilemma" by Clayton Christensen