Can't delete an ebook from ADE

If I delete an ebook from ADE (right mouse button - > delete from library) this ebook is deleted. But next time I open ADE, the same ebook shows up again in the library, that is: an empty image (thumbnail) of it. And I can not get rid of it.

How many times I press 'delete', next time this (grey) image of the (deleted) book shows up again. How can I permanently delete this?