Calibre.
It isn't the perfect replacement, but very close. And it is free.
I've used it with all my PDFs, epubs, CRBs (comics and manga), etc. When the metadata was missing or wrong, Calibre has the option to look up the correct information for book files. With my PDFs I can create a quick summary. With all the files keywords can be added to help sort files if you want.
Calibre has a dynamic, current community of developers who create useful plugins that can extend the features of Calibre in ways that Books for macOS can't. Calibre is also actively being developed.
The one big downside is that anything you purchased from the Apple Bookstore are DRM'd and won't play nice with Calibre. There are ways around it, but you'll have to hunt that down yourself.
Another downside is if you are looking for the sleek Apple aesthetic you'll be disappointed. There are a limited number of choices to change the icons, etc, but overall it has a dated look. Calibre is focused on being the Swiss Army knife of books, etc. The appearance aspect isn't high on the priorities list.
Calibre has a lot more useful features and ways to customize it to be the way you want, but it's just easier to visit the website, watch YouTube, and start out simple. Over time, you'll become so used to using it that almost anything else, especially Books for macOS will feel very limited and restrictive.
In summary, go with Calibre. Calibre is what Books for macOS should have been from the start. Not the enraging mess that it has been neglected into.