Copyright is automatic.
You wrote it, so you own the copyright.
You cement that with the copyright statement in the front matter, and what follows it.
The only reason for registering copyright in the US is if you intend to sue someone for infringement in the future, and want to claim damages from it.
That's all the registration is for. Without it, you can't claim damages even if you win the suet.
The reality is, most authors can't afford to sue anyone. When they do, the whole process destroys their ability to write during that period, and it's not worth it in the end anyway.
If someone pirates your book, most of the time they don't have it anyway, just the sample. Pirate sites are whack a mole anyway, and not worth the trouble, since they do nothing if you complain.
If your book gets put on a reputable site like Amazon, you fill in the take down notice form, and a month or so later after investigation, the pirated book will be taken down. Your publishing date for the first time on each book is all you really need to prove ownership. Once you tell the site where the original is, they can compare them, and when published, and the copy will be taken down.
I had this a couple of years ago, where someone put the same book up 5 times with different titles and author names, but using my series name as a base, and elements of the cover. Took a month, and listing all 5 infringements, but they were taken down. One of my fans pointed 3 of them out, and I found the other 2.
So registering copyright is not necessary, unless you want to go to court against someone.
However, if the book is pirated, the chances are, you will never identify who did it anyway, so never get them into a court.
The take down system does work. It's slow, but works.