In response to the last couple of posts, I've had several people write and say, "Book contracts seem a bit overwhelming. Why do I need to understand them?"
A book contract is a legal document, meaning that the words on the page will govern everything regarding your book for as long as it is in print. You wouldn't sign an agreement for a car or a house without reading it carefully and knowing what you're reading, so why do that with a book?
There are some ways to help make the task a bit easier:
1) You can sign on with a good agent. They ought to know more about contracts than you. (That said, one gentle warning: Ask questions. My experience is that some agents seem to know about contracts the way I know about auto mechanics. I mean, I know that a car has what's called an engine, and it's helpful if the engine works. Beyond that, I'm fairly lost… though I understand gas and oil are helpful ingredients, occasionally). You might want to ask any prospective agent some questions: Who have you done contracts with? How many have you done in the past year? What do you think is important in a contract? Where did you get your training? etc. A good, experienced literary agent ought to be able to help you understand and evaluate your book contract.
2) You could take your contract to a lawyer. They will either charge you a flat fee to review it, or charge you by the hour to pick it apart. A good lawyer can help dig into all the words and numbers, and figure out how to help revise them. My advice is to be very careful that you're working with a lawyer who knows something about publishing law and (hopefully) intellectual property rights. Your average lawyer who is doing mortgage closings and setting up grandma's will won't be that helpful. You want someone experienced, who knows how to read a publishing contract. They'll usually charge by the increment (and my experience is that they'll try to keep the clock moving, so put a cap on the time.) A good lawyer may charge more than the 15% commission you'd pay to an agent, so make sure you get what you're paying for.
3) You might take your contract to a contract evaluation service. Several people run these, including Sally Stuart and Susan Titus Osborne. I've seen the work both these folks do, and I thought they'd done a good analysis of the contracts they evaluated. For a couple hundred bucks, you get back a knowledgable response on the big-picture stuff that will give you some context and allow you to go back and negotiate a better deal for yourself. They won't do the negotiating for you, but they'll certainly get you on the right track by telling you what to expect and what common industry standards are.