Apple Books won't download any books/files saved in iCloud only on Mac (macOS Big Sur) I can't download any books/files I saved with iCloud in the Apple Books app on my Mac anymore (it worked before though). Books from the Apple Books store will download fine. I can also download all books saved in iCloud with iOS without problems.
There is no error message, instead the Download will apparently start as indicated in the sidebar:

However, the download will never process at all (again, every other download works without issue).
Things I tried that didn't work:

Rebooted the system
Logged out and back in with the corresponding Apple account
Disabled and reenabled syncing for Books in the iCloud settings
Removed all ~/Library/Application Scripts/com.apple.iBooksX.* folders and rebooted
Removed ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks folder and rebooted
This seems to be related to the storage optimisation feature for iCloud (maybe it is somehow stuck or blocking these downloads). It can be found under System Preferences -&gt; Apple ID -&gt; Optimise Mac Storage. After turning the future off and on all downloads in Apple Books now work again.