[Adoby]

The last page read, using the Calibre Reader, is stored inside the epub. The epub is updated only when you close the Calibre Reader.

You can verify this by opening the folder for a book and see how the modified time changes when you close the book, after flipping a few pages in the Calibre Reader.

I would assume that this would trigger a sync TO the cloud. And that, on the other devices, trigger a sync FROM the cloud. However it is possible that these syncs doesn't happen immediately. I assume that this can differ between operating systems and file systems, if a file change can trigger a sync event. Or if the cloud software make use of this or just poll for changes at certain intervals. I assume that you can force a sync manually?

Warning: Make sure that all your devices have the same case sensitivity. For instance all Windows or all Linux. Otherwise your Calibre library may slowly deteriorate with orphan books and database entries. The reason is that some operating systems / file systems don't care about UPPER lower case in file names, but some do care very much.