Only one address book? I have five. In normal use, if you write to somebody who is not already in your Address Book, their email address will be added to Collected Addresses. You can change the settings for this, and either stop Thunderbird from collecting addresses in this way, or tell it some other address book to use. Personally, I review what is in Collected Addresses periodically. If there is a name there that I thought was already in my Address Book, I will investigate to see what has happened, and add this address to the relevant address book if necessary. There may also be a few email addresses that I am not likely to want to use again, so I delete those. Collected Addresses has occasionally been valuable in that it has captured an email address (maybe one copy-and-pasted from somewhere else) that I hadn't thought to add to my Address Book. OTOH it also collects addresses I won't want to use again, so I am happy for them to land in Collected Addresses and not pollute any of my working address books. You can adjust the usage of Collected Addresses here: Tools|Options|Composition|Addressing Note that you can't eliminate the Collected Addresses address book; it is "hard coded" into Thunderbird and if you remove its data file it will be regenerated. So the best I can offer you is an empty Collected Addresses.