I would suggest that instead of right clicking and selecting add to address book you click the star to the right of the email address. If the star is empty it is not in any of your address books. If the star is filled with a color it is already in one of your books. Clicking a filled star opens the contact edit dialog box and you can select the book you want this contact to be in. So click the empty star once to add it and it will default to the personal book. Click the filled star and from the edit dialog move the contact to the desired book.

Thanks, Airmail.

It looks like, essentially, all you have to do is double-click the empty star to get a dialog that lets you select the destination address book. It would be better if you could change the default to a given address book, but I can't really complain that a double-click (as opposed to a single) is too much work. :)

You could perhaps try the card book add-on. It is an address book replacement and it appears to use your default address book to add new contacts.

https://addons.thunderbird.net/en-US/thunderbird/addon/cardbook/

Thanks, Matt. I'll probably stick with the double-click, but the add-on does look interesting.