This a pane at the left of the space where you type a new message?

If so, it's called the Contacts Sidebar, and in TB38, it defaults to "All Address Books" and it'll be showing you a composite address book, made up of all your address books including one you may not know you have, called Collected Addresses.

If you click on "All Address Books" in the Contacts Sidebar, you'll get a drop-down list where you can choose any one of your address books. Personal Address Book may well be the appropriate one for you.

I don't use the Sidebar. I just start typing addresses or names and an autofill feature offers matches from the Address Book.

When this topic comes up, I often ask about the thought processes behind working from a list of names. I guess there are use cases where you are sending to a large number of people, all of whose names you couldn't possibly memorise, so a list might act as an aide-memoire. (But there are better ways of dealing with such a situation; e.g. dedicated address books, Mailing Lists, mail merge utilities.)

Another use case might be the kind of person who indiscriminately copies amusing (supposedly!) messages to all their acquaintances.

In my own case, I write to people, and people have names, and their names are what I know them by. The number of people I write to in reference to a particular topic would be small and the addressees would be specific to the context. So it seems natural to me to address a message by naming the intended recipients. And it's more efficient to have the computer search my address books than do it myself.

To manage what goes into Collected Addresses, look at

Tools|Options|Composition|Addressing

Oberkassel said

Most of the new entries appear to be people I've either written to or received mail from. My address book is last name first. Many of the new entries were the same names but first name first. Something is obviously scanning my all mail file and adding contacts to my list. If I want to add a contact, I'll do so. 'Don't want it to be done for me unless I ask.

Thanks for your help. Spot-on. Problem solved.