Some books scrambled up letters

Hi there,

I am new to this whole thing, so please excuse me if I do ask some rather basic questions.

I have a ebook-reader here (tolino) and I am trying to import epubs that are stored on the reader. Some get correctly imported and I can read them, some of them cannot be read: I only see a scrambled up letters and characters. The same when I try to read them directly on the device. Now, I wonder what this may be. Can somebody help me on this please?