As an alternative, you can right click the book in Calibre and choose "Tweak Epub" and then "Explode Epub". This will take you to a temporary folder where you will find all the bits of your book. Open the contents folder and drag it into a "new book" in Mobipocket Creator, add the cover and fix the meta data. Then you can "Build" a prc version of the book. Sounds complicated, but it's not, takes just a couple of minutes.
cheers
|