You can delete book in library by:
(1) At Shelf View, doing a long press against a book cover, selecting the books that you would like to delete (multi-deletion, this is for Android devices running on Android 3.0 or above only) and then selecting "Delete" icon (for some devices, the “Delete” icon will be under the overflow menu).
(2) At List View, tapping on the "More" icon on the right of a book and then selecting "Delete" icon.
Please note that multi-deletion is available in shelf view only.
Notes: please do not delete books by removing the files from the storage of your phone/tablet. Otherwise, the actual file will be deleted, but the metadata of the file will still remain inside the Aldiko Book Reader. In this way, when you try to click on the book in the library of the Aldiko Book Reader, you will see "could not open" error message.