Can't sync all iBooks from mac to other devices I have done everything possible, starting from reading this :
https://support.apple.com/en-us/HT202759

I have set the sync option on the mac , and also on the iPhone/iPad.

When I buy a book outside the Apple store, i put it on my mac, then I would like to be able to see it on iPhone/iPad iBooks , but I just don't see it.

It would be wonderful if I could start reading on the mac and move to the iPhone.

Is it possible ?
Yes, it's possible.
  
  The article you linked to applies only to books from iTunes Store. In
  order to sync your books and PDFs outside of the iTunes Store you need
  an iOS device running iOS 9.3 or later and/or a Mac running Mac OS X
  10.11.4 (El Capitan) or later. You may update from App Store or iTunes.
  
  Also check:  iCloud Drive should be turned ON for all
  devices   Enable iBooks in iCloud Drive (Settings->iCloud->iCloud
  Drive->iBooks for iOS and Systemsettings->iCloud->iBooks for Mac) 
  Make sure that you have some free space for the books on iCloud Drive
    Make sure you are signed in with same Apple ID on all devices
    Make sure that "Hide iCloud books" are turned OFF
  
  More information:
  https://miapple.me/ibooks-sync-pdf-files-ebooks-via-icloud/