How do I stop the covers of certain books (not all books) appearing in the Purchased Books collection on iBooks?

There are books showing in the Purchased Books collection on iBooks which I do not intend to read again, and they are just getting in the way of other books on there.

I am aware you can hide all purchased books via the settings, but I only want to hide some of them.

I have successfully hidden them from the purchased books section of itunes, and have synced/backed up the phone by wire and iCloud since then, but the books still appear in the purchased books section of the phone...........