iOS - Library Location

The following information assumes that Apple Books syncing with iCloud Drive is disabled!

Apple Books for iOS stores its data in the Books directory on an iOS device.

[ios-device]
 │
 ├── Books
 │   ├── Managed/
 │   ├── MetadataStore/
 │   ├── Purchases/
 │   ├── Sync/
 │   ├── Books.plist ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ Books data
 │   ├── com.apple.ibooks-sync.plist ╌╌╌╌╌╌╌╌╌╌ Annotations data
 │   ├── 376FAA7E4CF81729.epub ╌╌╌╌╌╌╌╌╌╌╌╌╌┐
 │   ├── 669FEE1FFBB29D81.epub              ├╌╌ EPUBs
 │   ├── F788455723912C6D.epub ╌╌╌╌╌╌╌╌╌╌╌╌╌┘
 │   └── ...
 └── ...

Both the EPUBs and plists are stored in:

[ios-device]/Books

The books plist is located at:

[ios-device]/Books/Books.plist

The annotations plist is located at:

[ios-device]/Books/com.apple.ibooks-sync.plist

See iOS - Access Library for more information.