I am asked from time to time if a client needs to copyright their book. My usual answer is that will depend on one thing; if you are going to sell your book as an income source for many years to come, then you will want to get a copyright. If not, then you won't need to bother with a copyright. Many genealogy books or family history books have limited interest to close relatives and don’t need to have a copyright. Keep in mind that obtaining a copyright isn't very expensive and doesn’t take much time, so if you are wavering on this, then go ahead and copyright it.
Here is a link to the U. S. Copyright Office web page for frequently asked questions. If you don’t find your answer here, then let me know and I’ll see what I can find out for you. http://www.copyright.gov/help/faq/
Email questions and I’d be happy to help.