Copyright is established the moment a book is written, explains the U.S. Copyright Office. Registration through the office adds additional protection. These are the only two ways to establish copyright in the U.S. The idea of a "poor man's copyright" exists in popular culture, but is not legally valid.
Copyright takes effect when the content is placed in a fixed medium, as the U.S. Copyright Office points out. For a book, this usually refers to written text, but it may include video or audio recordings. Though copyright registration is not required, it is highly recommended. Registration is required to file any copyright infringement lawsuits.
The "poor man's copyright" involves an author mailing his work to himself through the post office to prove a date of creation. The U.S. Copyright Office explicitly states that a "poor man's copyright" does not provide any protection under U.S. copyright law.