I recently reread Vonnegut's "The Sirens of Titan" and was really surprised to notice to what degree Adams drew inspiration from that book. He did acknowledge it as an influence [1], but some ideas my naive self had attributed to Adams are closer to well-done rearrangements/retellings of elements from Sirens of Titan (which certainly wasn't written in a vacuum either) in a different comedic voice. Both works are fantastic, though.