I highly recommend John Harrington's book, "Best Practices for Photographers." Though it's aimed more towards the established editorial/advertising/commercial photographer, there is tons of great stuff for RE shooters as well. Harrington is a real-deal pro photographer. Especially revealing and informative are the book's many "case studies" covering a wide variety of topics where Harrington includes actual contracts and e-mail conversations with clients. His pricing policies and thoughts on getting paid what you deserve are enlightening. Example: instead charging late payment fees, he charges them up front and then discounts the client if they pay within 30 days. There are lots of little clever, but important, negotiating strategies like this in the book.
Since Aaron's original recommendation this book has been updated to the second edition. This book is chucked full of just all around good business advice. I highly recommend it.