Then don't compare it to Music app, compare it to Contacts, where you can't see where the library is.
But this doesn't matter. How you think things should be is interesting biographical information, but that is not how Books.app works. The developers have designed it in the way they have. Your options are simple: use the App as it is designed to be used or use an app that works the way you prefer. Using the app is not compulsory and there are other options. What's not an option is to alter how the app works.
Oh and you can transfer books to a new computer via iCloud, quite seamlessly.
Calibre is an excellent open-source manager for a library of books - I have some 4k books in two libraries managed by it. If you want an app that syncs across devices then check out Yomu, it's inexpensive. And there are more options than that again.