I am trying to free up space on my iPad by deleting books in my library I have read. I am following instructions as given to delete and hide but the book(S) remain in the library. Can anyone help me please?

As the above link says:

If you can't delete an item

• Hide a purchased book or audiobook: If the iCloud icon  appears next to an item, it's stored in iCloud and not downloaded on your device. Hiding the item removes it from your device. To access your hidden items, tap the Sign In button  or your photo at the top of the screen, tap Manage Hidden Purchases, then sign in with your Apple ID. Learn how to hide and unhide purchased books and audiobooks on your Mac.
• Turn off synced books from a computer: If you used your computer and a USB cable to manually sync books and audiobooks to your iPhone or iPad, you can turn off syncing to remove those items.