[stumped]

while we wait for official support...
1. I put a copy of my whole calibre library into pcloud
that's 3.25Gb but p give a generous 10Gb free allowance ( well they advertise 10 but for soem reason they seem to have given me 9 and are holding back with the other 1...) . I simply dragged calibre library from my documents to the virtual pcloud drive on PC and left it to do its thing.

2. I had already installed the pcloud android app onto my tablet

3. now in the tablet app I can see all the library files but I am pretty sure I have not lost 3+Gb of tablet storage. When I go to ../android....pcloud with xplore it says zero bytes used. I think it is a virtual system and data files will be pulled from cloud on demand. Also, the tablet did not go into any hang about while I download 3Gb frenzy

3. next I tried configuring CC to use a cloud library in internal storage to see if I could navigate to pcloud that way but I could not get it pointed at anything that worked...

4. So instead, I just browsed in my pcloud with their android app, navigated within the calibre library folder to find an unread book, selected open with moon reader and voila - I have it open and readable.

A peek at moon's book information tab says the moon has it stashed in an attachments subfolder of a main moonreader folder , as an epub.

( that for me is inside my books folder because I think I assigned that location to moonreader ).

so moon reader has its own local copy of the book and if I delete it, via moon's recent tab, the pcloud library copy should be unchanged...yes.