Where can I delete ebooks from Adobe Digital Editions definitely?

Hello

I'm sorry, if my english isn't very good, but i have a question and I saw that I have to ask here and normally I speak german.

I downloaded the Adobe Digital Edition to read and download ebooks from the library. But if I want to delete, nothing happens. If I click on delete, they disappear, but next time I open the Adobe DE again, the ebooks are there again too. I deleted them on computer RAM and ROM and deleted them at Adobe too.

Where can I delete them definitely?

Thanks for your help!