A pre-Discworld SF spoof, The Dark Side of the Sun sees Dom, scion of the hideously rich Sabalos family who rule the wet planet of Widdershins, going on an interplanetary quest for the origins of the Jokers, the first intelligent life in the universe.
It is, essentially, a romp: think a less subtle and less funny Hitchhiker's Guide to the Galaxy. There are some interesting ideas - it features a version of that old SFnal chestnut, "what if we could predict the future using science?", a la Foundation - and the various personages we meet are suitably off-the-wall (a sentient planet, a cannibalistic race whose change of sex during their life-cycle was probably more off-the-wall in 1976 than it is today, a security officer who runs checks on himself), but a little-known comedic gem this is not. It lacks the wit and perception of the Discworld novels, at least for me.
Completists will probably enjoy it though - both Hogswatchnight and Soul Cake Night feature in the book, and the Joker storyline definitely has echoes of The Long Earth in its quest for transcendence. Not as disappointingly bad as the later Discworld novels (Snuff, Raising Steam) - the story at least works on its own terms.