Deleting address books does not remove them from the file system but they remain as files creating clutter on file system: ~/.claws-mail/addrbook/addrbook-000003.xml.bak ~/.claws-mail/addrbook/addrbook-000004.xml.bak ~/.claws-mail/addrbook/addrbook-000005.xml.bak ~/.claws-mail/addrbook/addrbook-000006.xml.bak
These are not address book files, are backups of the addressbook files, which may save you some headache in case of accidents. If you don't want backups you can make a trivial shell script which reviews your ~/.claws-mail/addrbook folder and removes those addrbook-*.xml.bak files which don't have the corresponding addrbook-*.xml file. Put it under cron for automated deletions. As a general idea I don't think is a good idea that the application making backups is also able to delete them, but, anyway, this would be a RFE not a bug.