How to delete books from iPad when 'Share Series' is the only option?

If the only option when tapping on the ... dot menu is share, that means its a group of books for suggestions which may not all be on your iPad to be deleted.

If you tap on the group to open it, the ones you can delete from your iPad will have another ... menu. If it just shows a price, you don't own them and they are not actually on your iPad, they are only suggestions form the one you do own, so there's nothing to actually remove.

For those you do own and are downloads, you should get a Remove option and then an option to either just remove the download or remove the book entirely from your Library.