I've got a number of free books which I've converted using calibre to the mobi format. These work just fine when I upload to my kindle. However, when I send the same books to my new nook touch some (not all) come up as unknown file formats.
So, is mobi not the best format for nook? In this case I'll have to convert my books to epub as well? I share a lot of books between the 2 devices (my wife uses the kindle, I get the nook 'cause it's newer

). Suggestions welcome.