I am getting a file format not supported message when i try to add an ebook to ADE. What can i do?

Ok, I figured out how to add ebooks to the ADE library, but when i dragged the ebook to the library, there was a message that said the file's format is currently not supported.  I tried a couple other ebooks with the same results. Is there something i can do?  I have windows 7 and intel core 13.