iBooks sync problems - Mac & iPad

This isn't a new problem, it has been going on for over three years but it gets more and more irritating over time.

Basically, in iTunes on my MacBook Pro running the latest version of El Capitan (not really relevant as it happened with Mavericks and Yosemite - I only updated to El Capitan to supposedly take advantage of the better integration with later versions of iOS), under my iPad's settings I have 'all books' selected to sync but this never happens. Sometimes, I can chooses selected books and laboriously make sure that every book is selected but that does not ensure that they will all synch.

I have deselected iBooks from the iCloud Drive setting in System Preferences > iCloud because most of the books I want to synch are PDF equipment manuals for things like cameras, amplifiers etc. and very often, when I need them, I find that they aren't on my iPad despite having somehow synched them and referred to them previously.

I can't understand how an request as simple as synch all books can be so difficult to implement.

Can anybody help, please?