If you write something, you own the copyright, period. There's no registration process or anything like that. If you made it, it's yours, legally. And the only process involved to exercise your legal rights would just be proving that you're actually the one who made it.

Of course, none of that makes it certain that no one will claim it as their own or use it for something you don't want. As a general rule, just assume that anything you don't want used in a way you don't like simply shouldn't be put out into the public at all, regardless of what kind of license you package with it. If you're an average person and not a billionaire good luck exercising any kind of legal rights for intangible stuff like written words.

It is generally a good idea to include with anything you put out there some kind of license, which could be as simple as a .txt file that says "Made by [name], free to use for xyz purposes with abc caveats"

For a book stuff like that can go into the first or last couple pages that usually include all sorts of random boring information and publisher credits and whatnot