I was wondering about the zip thing too - wasn't certain if the debug messages definitively showed zip or not.
Manually copying a folder into the book's directory isn't the recommended way to go, though apparently it worked for the OP in this case.
Generally the recommended way to add an html book is to just grab the root html file (which in turn has href links to everything else) to Calibre's main window. Calibre will grab everything and create an appropriate zip for you. (you also need the html to zip plugin enabled, though this is default) If you created your own zip manually it probably wouldn't have worked, as Calibre places an OPF and some other items in the zip.
|