Here are some legal areas where you need to be careful--and may need to seek professional counsel.
Taxes. You are more likely to encounter legal difficulties due to a dispute over your taxes than for any other issue, especially if your book is a huge financial success. Don't play games with the tax collecting agencies by trying to hide your income or exaggerating your expenses.
Copyright infringement. Understand that you cannot use the words or illustrations of another person without their express, written permission. There are exceptions, of course, and something called "public domain" for older works that are no longer protected. But except for brief quotes to support key points, be legal and get permission.
At the very least, make at least three attempts to obtain written permission to use another person's copyrighted work. If you receive no answer, at least your written record will give you or your attorney a better chance of showing that you took reasonable steps to obtain such permission. It is also some reassurance when you receive no reply that the copyright holder will probably not raise a copyright issue with you.
Breach of contract. You may feel you've been short-changed, and perhaps you have, in your publishing or book production agreement. Nevertheless, the written agreement you have signed generally binds you to its terms. This is especially important if you are dealing with a vanity press. Proving to a court of law that a "reasonable person" would not understand what you are getting into is a tough road. Best prevention: seek professional counsel before you sign. Life is too short to waste money on disreputable book production services.
Online legal advice. We suggest a visit to the Publishing Law Center. You can sign up for a free email newsletter and benefit from tips and suggestions by an attorney who specializes in publishing law.