[petrjahoda]

1. You have to set up the Calibre connection and Calibre Mapping.
2. You need to have some books synced from Calibre.
3. You need to have made some changes to those books (read some, change the star rating, etc.).
4. The button then appears when there is something to sync (the '...' button will also turn blue to indicate that something has changed and can be synced back to Calibre).

IMPORTANT: Calibre requires a user to be created in 'Sharing over the net' for writing back to Calibre to work.