Why isn't iTunes recognizing new epubs added to Books on Macbook?

I have a total of 1,667 books added to the Books application on my Macbook. 127 of them are recently added. When I connect my phone to iTunes to sync the new ones in, iTunes only says there are only 1,540 {https://drive.google.com/open?id=1Pwe877D8BCKm5esYFQDKqJ98UF6FnWwE} books and doesn't sync any new ones into my phone. When I restart iTunes, it then says that there are only 281 books instead {https://drive.google.com/open?id=1jMrwF90rtFoxqz3NEvcv5un8ki8YOfls}. When I restart iTunes again, it goes back to 1,540. I've been doing this back and forth for the past hour and I'm getting frustrated. My phone (through iTunes) says it only has 1,540 books in it as well from my previous sync. Music syncs fine, as well as Movies; it's really just Books I'm having a big issue with.

I don't use iCloud for Books unless it's for sharing bookmarks, collections, etc. Across devices because I've found that the books have a tendency of disappearing when I'm not connected to the Internet. So, only bookmarks, collections, highlights, etc. Sync through iCloud.

Why isn't iTunes detecting the new books I've added to Books? How do I resolve this problem? Everything is up to date.

SYSTEM INFORMATION (Macbook)
macOS Mojave
Version 10.14.6 (18G95)
Macbook Air (13-inch, Early 2014)

SYSTEM INFORMATION (iPhone 6+, 64gb)
Software Version 12.4.1

ITUNES version 12.9.9.5
BOOKS version 1.19 (1645)

In macOS Catalina iTunes doesn't exist anymore so it doesn't matter

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