There are lots of helpful and motivating books, but here it is some of the free, nice stuff one can read online:
I second all of the books mentioned above, one of my favorites is REWORK, Founders at Work and The Art of the Start by Guy Kawasaki. All of these books are easy to read and there are tons of things to learn from them.
Another of my favorites is Peopleware by Tom DeMarco and Timothy Lister. It is an excellent book and you'll learn a lot from it.
"Switch" is a really motivational business book. However, it is more appropriate if you are trying to embrace and welcome the art of CHANGE. -Elle
Atlas Shrugged really made me passionate about creating something I can call my own.
Honestly, I get my most motivation out of reading about others succeeding.... so for example I loved the new Facebook book, The Age Of Facebook and the new Google book. Reading their stories really moves you.
ReWork and Getting Real by 37 Signals
Founders on Work by Jessica Livingston
Paul Graham's Essays
Joel Spolsky's Articles
Mastering VC Games by Jeff Bussgang
I am partial to A Good Hard Kick in the Ass: Basic Training for Entrepreneurs by Rob Adams. It is all about implementation and execution.