Can't transfer PDFs & eBooks from iMac to iPad 3 on iOS 6 I see there are loads of questions on this general theme, but I couldn't find one that was precisely . my situation.  If I just missed it, someone please point me to it.

I have an iMac running Sierra and iTunes 12.8.0.15, and an iPad 3 running iOS 6 (and I do not wish to upgrade it at all).  I have a number of PDFs and epubs that I used to be able to transfer to the iPad via iTunes, but that no longer works.  The books have also all been deleted from the iPad when I tried to do a sync from iTunes.

The "Books" option in iTunes has suddenly vanished before my eyes today (the one under "Settings" in iTunes).  iCloud Books also does not appear to work since even if Books are enabled in iCloud on my Mac, they don't appear on the iPad.  Not sure if iOS6 even supports iCloud books... I thought it did but could be wrong.

Anyway, bottom line is I simply want to be able to manually download my books to the iPad since I use it primarily as an e-reader these days.  Is this possible and if so, how?
OK I have absolutely no idea why, but today everything is working as expected.  On a lark I decided to fire up iTunes once more and see if I could get this to work, and the Books option is back, and all my books from iBooks, show up and will sync.  Go figure.