Deleting books from ipad

I went into Amazon and deleted two books I read from my content list. Went back to my ipad and they are still there. Held my finger on them to delete, but delete is not one of the options that pops up. I get "Add to a collection", "Download", "Recommend" and "Book Details". When I try then to "Download" it tells me I don't own the book I am trying to download. So there are 2 books I read and can't delete. Even after I sync the ipad to my Mac.