It could be a nice feature if it would be possible to add the same contact to several address books.
It could be done this way:
In the "Add contact" window - the combobox with address books should be changed to multiselectbox (this would let us add a contact to several addressbooks).
In the "email composer" after clicking addressbooks there could be added a new tab with addressbooks or some other widget which lets a user choose an addressbook.
To sum up:
1. The contact should be able to be added to several addressbooks (the same contact so when it is updated - the changes are visible in all addressbooks).
2. There should be a possibility to send an email to all conatcts from the addressbook by choosing only the addressbook as a receipment.