Books keeps syncing to iCloud despite repeatedly turning off syncing.

I'm at my wits end. I have several GB worth of ebooks in Apple Books that I want to keep on my Macbook's local drive. Lately, I've been getting daily notices that my iCloud drive is full and lo and behold my Books has synced to iCloud. No matter how many times I go to System Preference > iCloud > Show More Apps > Books and uncheck it, I go back in and find that it has re-activated itself. And of course every time I uncheck it, it removes all my ebooks from not just the cloud but my local drive. What kind of mental torture is this?