$book->setIdentifier("http://$_SERVER[HTTP_HOST]", EPub::IDENTIFIER_URI); // Could also be the ISBN number, prefered for published books, or a UUID.
//$book->setLanguage("en"); // Not needed, but included for the example, Language is mandatory, but EPub defaults to "en". Use RFC3066 Language codes, such as "en", "da", "fr" etc.
$book->setDescription(_("Some articles saved on my wallabag"));