Books NOT Syncing Between macOS and iOS

Both my iMac (macOS 15.1) and iPhone 12 Pro (iOS 15.6.1) are signed into the same Apple ID and iCloud syncing is enabled for Books.

Unfortunately, when I download a new ePub and add it to Books (and a Collection within Books) on my iMac, the Book does NOT appear on my iPhone even a week later.

What should I do to troubleshoot syncing Books from my iMac to my iPhone?