Are they showing up in your library or elsewhere on the edge? Usually, there are two ways to delete things depending on where they're found. You can long-press the book and slide your finger over to "delete". The other option is to look for a check box in the upper right portion of the screen. Tap that, and unchecked boxes will appear on each book. Check the ones you want to delete, and click "Delete" at the bottom.

Hope that helps!