Bellow hit the bestseller list in 1964 with his novel "Herzog". Bellow was surprised at the commercial success of this cerebral novel about a middle-aged and troubled college professor who writes letters to friends, scholars and the dead, but never sends them. Bellow returned to his exploration of mental instability, and its relationship to genius, in his 1975 novel "Humboldt's Gift". Bellow used his late friend and rival, the brilliant but self-destructive poet Delmore Schwartz, as his model for the novel's title character, Von Humboldt Fleisher. Bellow also used Rudolf Steiner's spiritual science, anthroposophy, as a theme in the book, having attended a study group in Chicago. He was elected a Fellow of the American Academy of Arts and Sciences in 1969.