I don't use Calibre for this (but I do for epub and pdf ebooks). I use Plex and use Plexamp for my music collection. For audio books I used to have Chronicle Audiobook Player installed (which is open source, but not actively maintained) but am now trying out Listen Audiobook Player (which is not open source as a heads up). Both of those allow me to download books wherever I am.

If you have no interest in running a Plex server neither of them are suitable for you, but I just wanted to mention this solution in case you're not dead set on using Calibre. Plex has a cost factor which may come into play (an obvious non starter for some). I have had Plex Pass for a long time so it was natural for me to use it, but might not be for you.

I have used several, the first I used was RedSquirrel which later evolved into the commercial Virtual Acorn. After RedSquirrel stopped getting updates, have mainly used RPCEmu.

The Shannara series by Terry Brooks, with the shortlived Shannara Chronicles TV series (it was not specified the show or movie had to be good...)