I had no idea! I’ve read the Hitchhiker’s books before, and I knew it was performed for radio, but I always thought the book came first.
As for the radio version, the Primary Phase is a delight. The voice actors are all top notch, especially Stephen Moore as Marvin the Paranoid Android, and the writing was so ahead of its time for the 70s (and the 80s and 90s, for that matter). I would easily pay 30 Alterian dollars a day to listen to this again and again.