Q: How to delete purchased books from IPad library?

You cannot remove anything from the purchased list in any of the stores on the iPad. You can hide the purchases but you cannot remove them - ever. You will need to hide the purchases in iTunes on your computer.

iTunes Store: Hiding and unhiding purchases

in iBooks, to remove them from your bookshelf, tap the Edit button, then select the ones you want gone so they have a check on them, then tap delete (upper left).

There is no way to permanently remove a book from the purchased history, but I found a way to erase all the history.

I change the country of my iTunes Store, and all the history is gone.

I haven´t try to change again to my previous store, I think they will appear again but I don´t know.

I hope this help