Robert Indiana kept a series of illustrated journals during the late 1950s and 1960s, in which he discusses the development of his work.

In his journal page for May 10, 1962, Indiana records that he "stuck to [the] painting table most of [the] day, only going downstairs [to] eat." The entry includes a sketch of The Sweet Mystery, with Indiana noting that he changed the "off-black diagonals [to] cadmium red light," and that the work is oil on canvas, and measures 72 x 60 inches. At the bottom of the journal page are sketches of the stenciled titles for three of Indiana's sculptures, Orb, Brow, and Zig. 

Indiana's journals also provide insight into the artist's daily life. Here he notes that he received a call from the gallerist Rolf Nelson in California, and describes "a great dock fire on [the] far side of Brooklyn, which filled [the] sky with a billowing cloud of smoke most of [the] afternoon" (an 8-alarm fire at the Sinclair Oil Storage Yard).