I updated Calibre to 0.7.40 today, and it is not showing any books in the device library.
It recognises that my Kobo is plugged in, and when I first connected it ran a job to retrieve the library, but nothing appeared. In the main library menu, it has checkmarks next to the books that I have transferred to my Kobo, however when I flick over to the device library screen it shows as a blank.
As an aside, it still has at the bottom of the screen, that I have 900mb 'ish free space, and when I added a book, this number reduced, I assume reflecting that the book was added to my Kobo.
Am I missing something really obvious here?