0.0.0.0 is pretty standard (take a look at the output from "netstat -an" on windows or linux (or mac?) for example) and generally means "any valid IP". That is, if a server is listening on 0.0.0.0:someport, any request to a valid IP bound to that computer on that port will be handled by that service. Similarly, if a service is accepting from 0.0.0.0, any valid incoming IP will be handled (in netstat, you'll see 0.0.0.0 when a service is listening, as once a connection is established between two IPs you'll see those specific IPs listed instead).
Calibre doesn't give you the option to bind to a specific interface/IP (at least not from the UI), so 0.0.0.0 is correct -- it's binding to any/all interfaces/IPs.