As a follow up to Lenny Campello's post on using contracts, I'm recommending these books with sample contracts and other legal information for artists. Now's a good time to start your contractual life.
These top two books are very helpful to Professional Artists.
The 11th Edition Graphic Artists Guild Handbook: Pricing & Ethical Guidelines
Legal Guide for the Visual Artist by Tad Crawford.
Business Letters for Artists by M. Stephen Doherty, User-friendly art business letters to customize to your needs.
Art Office by Constance Smith and Sue Vidders, Sample letters and simple forms for organizing your art business.