What you would have to do is set up either port forwarding on your router to pass port 8080 (or whatever port the calibre server is running on) requests to the machine running calibre. Or, put the whole machine in the DMZ which is much less secure. That said, make sure you password protect it if you are going to do this.
Check out this website for help setting up your router to port forward:
http://portforward.com/
BOb