This question came up earlier this week. It's curious to me that the forum
gets a series of posts on one subject more or less at the same time.
You can search the forum for the earlier post and its response, so you can
get the details of what the other person(s) experienced.
You can not delete ebooks en mass. You have to delete the ebooks manually,
using ADE's utility functions. Hover your cursor over the small arrow next
to the ebook's entry in Library mode. A drop-down menu will appear, and
one of the choices is to delete the ebook.