private function _getAddressBook($importType, $websiteId)
    {
        switch ($importType) {
            case \Dotdigitalgroup\Email\Model\Importer::IMPORT_TYPE_CONTACT:
                $addressBook = $this->helper->getCustomerAddressBook(
                    $websiteId
                );
                break;
            case \Dotdigitalgroup\Email\Model\Importer::IMPORT_TYPE_SUBSCRIBERS:
                $addressBook = $this->helper->getSubscriberAddressBook(
                    $websiteId
                );
                break;
            case \Dotdigitalgroup\Email\Model\Importer::IMPORT_TYPE_GUEST:
                $addressBook = $this->helper->getGuestAddressBook($websiteId);
                break;
            default:
                $addressBook = '';
        }

        return $addressBook;
    }