iBook books are not part of the backup.
Other books get saved in various ways depending on your settings. In Settings > iBooks there is a setting to turn on iCloud Drive. It only says it will save your PDF's, but I suspect it also saves non-iBook ePubs, but I'm not sure.
Under Settings > (your Apple ID) > iCloud there is a switch for each app which controls whether they get backed up. This might be what decides whether ePubs get backed up in iCloud.
On iTunes, your iBook library used to look the same as your iTunes library. You didn't back it up, you synced it just like you synced your music. I'm not sure how it works on Windows these days, but neither Music nor Books were ever a part of the backup. Same for the video library.
Basically, if it syncs, it isn't part of the backup file.
On a Mac the Books app (like Music) has become a separate app which can be synced with your iOS devices. I haven't direcly synced my iOS/iPadOS devices in a long time, so I'm not sure how it's done.
Here is an Apple article on the various ways to sync your computer and iOS devices.
Here’s everything you need to know about iPad, straight from Apple. This definitive guide helps you get started using iPad and discover all the amazing things it can do.
support.apple.com