For those who wish to continue using iTunes Apple have now released 12.6.3 which will continue to offer app store functionality and it includes iBooks
Users who have upgraded to 12.7 can apply this "retro" version
Apparently there will be no auto upgrades to this version so will suit many of us just fine
https://support.apple.com/en-gb/HT208079
I will now go back to my Calibre/iTunes sync combination to manage my book collection