If you're the owner of the copyright of the book, you have the exclusive right to copy the book, prepare derivatives of the work, distribute the book and display the book as you so choose. If anyone else does these things without your permission, you can take legal action against them. To take legal action you will need to register the book with the United States Copyright Office, but to copyright your book you simply need to create the work in a tangible form that is perceptible to another person.
Compose your book in a tangible, perceptible form. This means that you will have to write the book on a computer or on paper, or create a sound recording of it.
Make at least three copies of the book. Place at least one of those copies in a secure area, such as a safety deposit box or a safe.
Send copies of the book to at least one trusted friend or family member. It's a good idea to send the work to someone else in case your own copies get misplaced or lost.