I cannot recommend enough "Founders at Work" by Jessica Livingston.

It is a series of interviews of the founders of companies like Apple, Paypal, etc about the very very early stages of their start-up before they ever knew they would be successful. It is a great book to read both for motivation and for lessons learned.

You can read some of it here: http://www.foundersatwork.com/

Good luck with your start-up!