It is very frustrating trying to find bibliographical information for any Stamperia Valdonega publications: almost all attention is directed to the more prestigious hand press operation of the Officina Bodoni.

There is a 1992 Grolier Club exhibition catalogue, celebrating the centenary of Mardersteig's birth, which might contain the information you seek. The exhibition included books from both presses. The full title is The Officina Bodoni & the Stamperia Valdonega : an exhibition marking the 100th anniversary of the birth of Giovanni Mardersteig.

I wonder if it's this:
Ein Leben den Büchern gewidmet
Gutenberg-Gesellschaft. ; Stamperia Valdonega.
Mainz : Verlag der Gutenberg-Gesellschaft, Stamperia Valdonega, 1968
Kleiner Druck Nr. 84 der Gutenberg-Gesellschaft

A local library has a copy so I've ordered it; I should be able to post the original German in a few days (if I'm right that this book is the source).

Maxim,

Mardersteig's "Credo" statement from 1929 reads very different in Italian and German, if compared to that "New Yorker"'s 1970 quotation. Here it is in Italian:

"Cinque sono gli elementi del libro, ossia testo, carattere, inchiostro carta e legatura. Comporre con questi cinque elementi un tutto coerente e plausibile, non sottoposto alla moda, il cui pregio sia stabile e sciolto dal tempo; comporne delle opere affrancate, per quanto può esser dato a opere fatte da uomini, dagli influssi del capriccio e del caso, e degne dell’alto retaggio di cui siamo depositari e responsabili: questa è la nostra ambizione". (as quoted in Antonio Cavedoni's site: http://cavedoni.com/2007/02/credo)

And in German:

"Ein Buch besteht aus fünf Elementen, das sind Text, Schrift, Druckfarbe, Papier, und Einband. Aus diesen Elementen eine Einheit zu schaffen, die selbstverständlich überzeugt, die nicht einer Modeströmung dient, sondern zeitlosen Wert anstrebt, das ist unser Wunsch. Vom Zufall und Laune frei, soweit dies menschliche Bedingtheit vermag, kennen diese Werke nur das Ziel, sich würdig in das hohe Erbe einzufügen, das uns in Hand und Verantwortung gegeben wird". (from Wikipedia)

Another authoritative Italian source, by the way, is more similar to your quotation; it is taken from Il Polifilo publishing house site, who has edited a great Mardersteig book: http://www.ilpolifilo.it/storia_txt4.php, and it goes like this:

Per prima cosa servi l'autore,
cerca la soluzione migliore
per rendere comprensibile il testo.
In secondo luogo servi il lettore,
rendigli la lettura piacevole e facile.
In terzo luogo dai a tutto una veste attraente senza essere troppo eccentrico.

My translation:

First serve the author,
find the best solution
to make the text understandable.
Secondly serve the reader,
make his reading experience pleasing and easy.
Thirdly give the whole a charming look without being too eccentric.

Hope this helps.

The German text appears to be something else that Mardersteig said, not the quotation being sought. It begins: "A book consists of five elements: the text, the typeface, the ink, the paper, and the binding".

EDIT: Ah, here we are. First, find the German word for self-will in Google Translate, then search for it together with Mardersteig -

Als erstes diene dem Autor, suche die beste Lösung für die Wirkung seines Themas. Als zweites diene dem Leser, mache ihm die Lektüre so angenehm und leicht wie möglich. Als drittes gib dem Ganzen ein anziehendes Gewand, ohne zu eigenwillig zu sein.

Thank you for the search tip, John. Yes, this version is posted on Zeit Online Web page (»Wunderwerke der Druckkunst«). Now all there’s left to do is to find out the source (the publication title and the date) of this quote. I look forward to hearing from you, Craig.