By shadowing (not bi-directional sync) Your Library onto your cloud service.
A Calibre Library will get messed up if you try and RUN it from the cloud directly.
(A whole lot of file stuff happens with adding and modifying metadata. It must happen as a transaction. The cloud fight these kinds of things and any Library folder sync mode must be halted during the time calibre is running)
http://manual.calibre-ebook.com/faq....rked-drive-nas