This app also makes some kind of cache file. You can see this by opening couple bigger books and monitoring your disk space.
I didn't mean cache, I meant Library.
When you open an EPUB with iBooks, BookReader, Clearview, or Calibre, these apps add the EPUB to its own library (thus "importing" the EPUB). For most EPUBs that I only want to skim, the "always imports EPUB" approach is annoying. Thus my preference for Murasaki, which is just a reader and not a library manager. Makes it a faster app too. Of course, you can always just delete the EPUB from your library in any of these apps!