Copyright, a form of intellectual property law, protects original works of authorship including literary, dramatic, musical, and artistic works, such as poetry, novels, movies, songs, computer software, and architecture. Copyright does not protect facts, ideas, systems, or methods of operation, although it may protect the way these things are expressed.
Your book is protected by copyright as soon as it is “created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device.” This means your work is protected as soon as you press SAVE in a word processing program! For more on this, see the "Copyright in General" section of the FAQ page on the U.S. Copyright Office website.
You can register your copyright with the U.S. Copyright Office. Registration has certain benefits, such as creating a public record, which you may wish to take advantage of.