Sometimes an update will upgrade a dependency but not the application itself
because one is not available yet and a programme will not work for a time. I
used Calibre today and it works for me and both of my 10.04 installations
are up to date. I would try to update again and perhaps it will fix things.
Barring that I would re-install Calibre. Do not completely remove or you
will lose your settings including bookmarks.