What do you see when you open the Address Book proper? Very often what you're reporting happens because the email address is in the wrong field in the contact card.

Thanks for he reply. In the address book email addresses are in the field headed 'Email'. Contacts have been entered manually into the address book. When composing a new email entering the start of a name will pick up the email address and place it in the 'To:' field. So this part works OK. It seems to be just getting the contacts to list in the Contacts Sidebar which would enable use of the 'Add To ' functions (eg Bcc) at the bottom of the sidebar. A screen shot is attached. Note I have more than the 2 email addresses listed.

You expect to see all your contacts in the selected "Personal Address Book"? Not in Collected Addresses? Or some other address book?

I don't have the "category filter" option here. Is that from an add-on? What happens if you run Thunderbird in Safe Mode?

Running in Safe Mode shows all the contacts (even those with no email address, which would be superfluous) for the selected address book in the Contacts Side bar. So now to work out how to get this functionality into the 'normal mode'. The "category filter" has disappeared in safe mode. I have only recently installed TB and have not done any add-ons yet. My version is 52.7.0 (32-bit).

Found that the 'Card Book' Add-On had been enabled. Disabling this has fixed the problem. Thanks for your help.