For all the faded creepiness, the moment is worth revisiting, because mystics like Péladan prepared the ground for the modernist revolution of the early twentieth century. John Bramble, in his 2015 book, “Modernism and the Occult,” writes that the Salon de la Rose + Croix was the “first attempt at a (semi-)internationalist ‘religion of modern art’ ”—an aesthetic order with Péladan as high priest. In the years that followed, radical artistic thinking and obscure spiritual strivings intersected in everything from Kandinsky’s abstractions to Eliot’s “The Waste Land” and the atonal music of Schoenberg. In Yeats’s “The Second Coming,” the “rough beast” that slouches toward Bethlehem, half man and half lion, is no metaphor. Classic accounts of modernism tended to repress such influences, often out of intellectual discomfort. In recent decades, though, fin-de-siècle mysticism has returned to scholarly vogue....