The easiest way to delete a book from AUGEN BOOK is to connect it to your computer with USB and delete it the same as you would from any USB or SD card. Of course, if the book is already on the SD card, just move it to computer to do this.
BTW, if you were able to download a book via the Augen wireless, you were just lucky. It is SOOOO SLOOOOOW!
Since there is NO support, the Augen is best used the latest free Calibre ebook software and your computer via USB. Then, you can handle all additions and deletions through Calibre.