"Jours effeuillés: Poèmes, essaies, souvenirs". Book by Hans Arp, Gallimard, Paris, p. 571 (the book gathers almost all Arp's writings in prose and some important poetry), 1966.
"Aesthetics and the Environment: The Appreciation of Nature, Art and Architecture". Book by Allen Carlson, Routledge, London & New York, p. 118, "Abstract Art, Concrete Art", 2005.