It's because of your past purchases (deleted or not). If deleted they will not show in the library but are now counted towards your phones storage. Source: lots of destructive support from Apple over the last 8 hours. I say "now counted towards your phones storage - because in iOS 16 deleted book purchases did NOT count towards your phones Settings>general>iPhone storage.
iOS 17 counts towards storage data from your purchases, even the ones you made years ago and now deleted (they're all in Manage Hidden Books).
I have 3 devices with the same iCloud account signed in. On one device still on iOS16 and after deleting all content it has 30mb of storage, the iPhone on iOS 17 it has 1GB of data for Books.
Here's the strange thing. You can't get rid of it; even if you sign out of your Apple ID on the device!
Also If you delete the App it should delete all content - but on reinstall, it all comes back immediately - EVEN if you are not signed in to your Apple ID and/or you have iCloud>books syncing toggled off! So essentially, according to iOS 17 if you got the data on your phone it will stay for ever.
I think Apple is trying to tell you it's time to put that device in the draw and get a new (larger capacity) device. Sorry iOS has ruined your device.
The only known solution is to travel back in time and not use Apple Books
I'm on iOS 17.2.1