Locating the iBooks folder in iCloud Drive
Solution 1:
I've been trying to figure out the same thing. I did discover that in terminal you can navigate to the following directory to see your iBooks:
/Users/YourUserName/Library/Mobile Documents/iCloud~com~apple~iBooks/Documents
I have not figured out how to go here in Finder other than the method that you listed.
Solution 2:
You can open the directory easily, like that command in your terminal
open ~/Library/Mobile\ Documents/iCloud\~com\~apple\~iBooks/Documents/