Currently the LDAP address book implementation only allows for a global addressbook with static credentials set in the config.php (binddn, bindpw).
If a user has a private abook on an LDAP server that requires a user specific binddn/pw then there is currently no way to access, because the LDAP implementation in squirrelmail has no way for a user to specify these values on a per user basis. We would need a way to specify private LDAP options in the "options"-menu after logging in to squirrelmail.