J. Spiller, Paul Klee. Das bildnerische Denken. Form und Gestaltungslehre, vol. I, Basel, 1956, p. 529 (illustrated p. 452).<br>M. Huggler, "Paul Klee", in Künstler Lexikon der Schweiz XX. Jahrhundert, vol. I, Frauenfeld, 1958-1961, p. 529.<br>J. Smith-Pierce, "Pictographs, Ideograms and Alphabets in the Work of Paul Klee", in Journal of Typographic Research, vol. I, no. 3, July 1967, p. 229.<br>C. Kröll, Die Bildtitel Paul Klees. Eine Studie zur Beziehung von Bild und Sprache in der Kunst des zwanzigsten Jahrunderts, Dissertation, Bonn, 1968, pp. 37 & 223.<br>J. Smith-Pierce, Paul Klee and Primitive Art, Dissertation, New York and London, 1976, p. 146.<br>S. Wada, Paul Klee and his Travels, Tokyo, 1979, p. 133 (illustrated fig. 437).<br>K. E. Kramer, Mythopoetic politics and the transformation of the classical underworld myth in the late work of Paul Klee, Dissertation, Columbia University, 1993, p. 99 (illustrated fig. 62).<br>P. L. Kort, The ugly face of beauty. Paul Klee's images of Aphrodite, Dissertation, UCLA, 1994, pp. 153-164 (illustrated p. 336).<br>The Paul Klee Foundation, ed., Paul Klee, catalogue raisonné, vol. VII, 1934-1938, Bern, 2003, no. 7583, p. 481 (illustrated).<br>J.-F. Lyotard, Discourse, Figure, Minneapolis and London, 2011, p. 451 (illustrated pl. 16b).<br>