No. 3
The Millionaire Real Estate Agent
by Gary Keller

Although this book was written with the real estate agent in mind, the information is applicable to all solopreneurs.  Use it as a guide to build a strong foundation for your business.  It covers everything from marketing and managing a business, to leveraging your time and resources.  It truly is a manual for building a million-dollar company (if that's your goal).

No. 2
The 4-Hour Workweek
by Timothy Ferris

It's important as a solopreneur not to let your business control you, but rather to control your business. You have to keep a big-picture view of why you do what you do - what's the end goal (freedom, flexibility, financial independence?). This book will show you how to lose any preconditioned ideals of who you're supposed to be, how you're supposed to work, and how much money you're supposed to make. Free yourself from those limiting beliefs, and you'll be way more successful in business and in life.

No. 1
Book Yourself Solid
by Michael Port

By far, this is my absolute favorite business book. It is filled with ideas for marketing and promoting your business, and more importantly, it explains the importance of "bold self-expression" and only working with those clients most suited for you. I pick it up on a regular basis, just to see if I can further fine-tune my profile of an ideal client, as well as my marketing plan.

Use PaperBackSwap.com to trade books with other avid readers.  You'll only need to pay to ship your outgoing books (usually around $3 per book), which is much cheaper than buying them new.  When you're done, recycle them again!