The default is my bbdb addressbook. Using EUDC allows me to use LDAP at work
pretty readily, like this:

(setq eudc-server-hotlist '(("ldaps://ldap.somewhere.tld" . ldap)
                            ("" . bbdb)))

(setq ldap-host-parameters-alist '(("ldaps://ldap.somewhere.tld"
                                    auth simple
                                    base "ou=Users,dc=somewhere,dc=tld")))