Not a direct answer to your question about databases, per se, but have you considered whether this address book will need to be used by any other systems or system pieces? If it will, you might want to check out the LDAP schema, an LDAP server and one or more of the LDAP modules.

You didn't say what DB you'd be looking at using underneath your new address book application. What DB module will be best for you may be dependent upon what database you use.