Fortunately I got some help from @telega with this
 

---

There is a thick separating line/border in Simplemail between the folders Inbox, Sent, Drafts etc and the Address Book

If by mistake you click on the middle of this separating bar then Address Book will disappear from view

In fact the very same bar can be found now right at the bottom of the left page in the same column

Go to the bottom of the page and you can drag up the separator (with address book with it) or click on the separator et voila your Address Book is back

This is good news  !

Thanks @telega !

I have my address book back !