There were two radio series initially and the 1981 TV series covers season 1. There was rumour of a legendary third radio series, but Adams continued the story in book form with three further novels:
Life the Universe & Everything; Mostly Harmless; So Long & Thanks for All the Fish or as DA puts it, 'A trilogy in five parts'

.
Adams had been working on adapting the novels into radio plays up to his death in 2001 and they were subsequently broadcast afterwards. You can get all five series in box set & MP3 formats now. I have series 1 & 2 which come with a disc featuring an interview with DA in which he reveals the true origin of the '42' gag.
If I'm doing anything long winded, like decorating or out on a summer's day gardening, I usually whack the Guide on to listen to. Adams, you bl**dy genius!.