Under U.S. law copyright vests automatically in your book once it is fixed in tangible form: you don’t have to do anything. Even printing the © copyright symbol  is optional. Registration of your copyright with the Copyright Office does however enable you to act promptly in case of an infringement. Whether you think this worth the $35 fee depends of course on how you value your work. The potential for infringement might not be high, and you might well decide not to chase down anyone who “steals” your copyrighted material. One might regard such a theft as the sincerest form of flattery. However most publishers, I think it’s fair to say, do go through the step of registering their books with the U. S. Copyright Office.

If you do decide to register, this post by pbs.org will walk you through the process.