Seriously, running Catalina OS on the MacBook, and mounting the iPhone, and going to the Books tab, I only see 4 books that must have been from the time iTunes took care of the synching of books.

I have over 300 books in iBooks, and none of those (None purchased from Apple) show up when I try to sync either an iPad or iPhone.

I have purchased iMazing app during their sale to avoid this problem, but why cant Apple make this work?, Is the purpose of the BOOKS tab in Catalina when using an external device only to synch Apple Books?

Back to iMazing

If you have Books syncing via iCloud then the missing books are probably in a Books folder in the iCloud Drive.

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Not using iCloud, as that was a problem using two different Apple ID's when trying to get books to the wife iPad. Good thought, now that both my iPhone and MacBook are on the same Apple ID, maybe I should turn iCloud back on, to get it. Thanks, for jostling my memory.