So I use calibre as my backend client essentially. My library is managed through there, and I load my books in there as I get new books. It’s a bit clunky, but it’s reliable for what it does, and can even be set to auto tag your books and grab new covers for them

Calibre-web is what I mainly use to interact with it in my day to day uses. It’s a very clean front end that connects to my calibre server, and even has account management if that’s your thing. It’s hosted as a website, so I can access it from anywhere in the world. When I click on a book, I can either read it in the browser, or I can download it. Usually I just download them to my tablet and read them there as you would any other pdf/ebook. It’s a super clean way to manage a ton of books