A sync function is not that simple as it has to take account of issues such as:
- Tracking books that should NOT be synced to the device. I have far too many books in my library to be able to sync them all but I would not mind a very simple way to sync new books I have added to the library (unless I explicitly mark them as NOT to be synced).
- What if I re-convert a book - should that now be synced?
- If I delete a book from the device (which I do when I have read them) will calibre remember it has previously been synced and not put it back.
Issues such as these are not insurmountable but they all add to the complexity of a good sync solution.