I could not get it to work on my vintage Mac. iBooks (as it was then known) does not seem to interoperate with iCloud documents for Books running more current macOS.
I installed iCloud Drive on Windows but didn't see any obvious way of getting Books 'user documents'. It's just for sharing files and photos (and also share web site passwords via Microsoft Edge plugin Apple developed).
And it seems you can download, evict, and copy iCloud documents (including Books 'user documents') using command line.
The folder for Books user documents is
/Users/[user name]/Library/Mobile Documents/iCloud~com~apple~iBooks/Documents
(This same folder exists on El Capitan but it was empty, so apparently it is no longer supported on server side.)
(I suppose you can also copy files to this folder and iCloud daemon will upload them? Seems like it could be a simple calibre plugin to manage Apple Books content that is user-supplied.)
and the tool to control iCloud daemon download and un-download is called
brctl.
'man brctl' will show the options.
https://apple.stackexchange.com/ques...es-to-download
Note that ePub files are represented in iCloud as folders, just as if you had 'exploded' it. So in addition to copying, you need to have script to re-constitute it as an ePub file (with contents in the expected order).
As to @JSWolf's point, this folder only contains user content. Yet deDRM refers to another tool that can 'liberate' purchases from Apple Books but I don't know the current status of that tool or how it manages to work. I've made only a handful of such purchases (mostly just free things like user guides for Apple devices, developer documentation etc.), so it is not something I'm particularly curious about.
But I agree (even assuming it is possible) that it is probably more trouble than it is worth, especially for content that doesn't have DRM in the first place.