MacOS Apple Books: Where are imported books stored.

I have downloaded non-DRM epub books (Humble Bundle) to my download folder in my iCloud drive. I then import them into Apple Books.

Can I safely remove the epub books from my download folder after they appear to be imported into Apple Books.

Do I need to wait until the books synchronize with iCloud before they can be deleted, and if so how do I determine that the synchronization has completed?