Here is a list of books that I would recommend for those getting into the business, and for those already there.
Graphic Artists Guild Handbook of Pricing & Ethical Guidelines
Easily the most recommended reference book for creative professionals, and for good reason. It offers comprehensive information on professional ethics and business standards. Including pricing, business practices, contracts and legal issues.
http://www.gag.org/pegs/index.php
The Business Side of Creativity by Cameron S. Foote
A good companion book to the previously mentioned GAG Handbook, in comparison, more of a how to book. As the title suggests it discusses creative services from a very business oriented point of view.
How to be an illustrator by Darrel Rees
New book that discusses the subject through interviews with contemporary illustrators (including our own Sam Weber) and art directors. Relevant and informative.
How To Be a Graphic Designer Without Losing Your Soul by Adrian Shaughnessy
This book comes highly recommended, contemporary and relevant.
Artist's and Graphic Designer's Market
Children's Writer's & Illustrator's Market
Where and how to sell your work. These books, published by Writer's Digest Books, are a must have for freelancers or those looking for work. Comprehensive and up to date listings of art buyers including editorial, advertising, publishing, and more. Also includes interviews from professionals.
http://artists-market.com/
http://www.cwim.com/
Illustrating Children's Books by Martin Salisbury
Highly recommended not only for those interested in illustrating children's book. Clear and informative book that goes through the creative process of creating pictures for publication. Offers a very effective overview of how pictures are created including, a brief history, Drawing, Media, material, and techniques, Character Development, Concepts and Ideas, Getting Published.
How To Be Your Own Literary Agent by Richard Curtis
A good resource for anyone interested or trying to get through the labyrinthian world of book publishing.