I think calibre v3.0 is slated to have server-based write access.
At least, it was always one of those "one day when I rewrite the content server" things, and @Kovid is currently... rewriting the content server!
So if you are willing to wait ~few months, you might just get what you are asking for, for free.
Personally, my books start on the computer and make their way to my devices.