There's an automatic updater
floating around someplace in the forum for the 64bit version...
And the PortableApps launcher for Calibre also supports automatic updating but requires manual downloading of the portable package and that it is placed in same folder as the launcher. Be aware that you then also need to make provisions for the packaged version to be removed after upgrade or will trigger again on next launch (as long as the install package is there the launcher will keep 'updating')
Neither is totally automatic as the 64 bit updater needs to be launched separate to check for the update, and the portable versions needs manual downloading...
But both have the advantage of not being totally 'creepy'
FWIW: Both methods can be adapted to personal needs as both come included with their respective NSIS scripts