Dropbox is a handy way of syncing this, including your whole library too!
(Just don't leave it syncing while calibre is actually running.
)
You can also use command-line scripts with tools like rsync (
see linux example here) which could be modified to use a windows batch file with rsync for Windows -- assuming you are techie enough to do so, I guess.
It is, of course, much easier to do on linux.
