Love Marvin.....such a beautiful interface! But one request to make it more "beautifuler" ;-)
Honour the book cover art. Though Calibre may have the correct (or any!) cover art (custom, etc), Marvin wants to display the art that is embedded in the file, sometimes that means no art. That's plain ugly ;-)
PDF support would be nice, I have a lot of text books in PDF, but that's less important right now!
Keep up the damn fine work!