They are both built in and you're not invited to remove them. If you do, by deleting the underlying data file, it will be quietly re-created.

If you send a message to an address that is not in your Address Book, by default that address will be saved into Collected Addresses. You can change this to use the Personal Address Book instead, or disable the collecting function altogether.

So you can't remove either of these address books, but you don't have to use Collected Addresses.

I have at least six address books. I can't imagine reducing it down to one with all my different types of contacts jumbled together. I periodically review what's been put into Collected Addresses, file the useful addresses and delete all the rest. I wouldn't want the collecting function to be allowed to pollute any of my working address books.

Thank you for the background info. If I'm obligated to more than one address book, how do I default to the Collected Addresses when I open a new message window? 90% of the people I correspond with are in the Collected Address Book.

Thank you once more.

If you mean that you want the Collected Address to appear every time you open the Contacts Sidebar in the Write window, select CA at the top of the sidebar, and TB will store the setting for the next time you start a new message.

If you mean that you want address autocomplete to search CA only, when you type an address in the To: box, install the MoreFunctionsForAddressBook add-on.