Douglas Adams died ridiculously young and didn't write anywhere near as many books as we'd like. But he did have ideas for a sixth Hitchhiker book shortly before he died, and that book, And Another Thing Ã?Ã?Â¢Ã?Ã?Â¦, was written by Eoin Colfer. Okay, it's not Adams, but it's a worthy conclusion to the series.
As for Adams's own work, you really don't want to miss Dirk Gently's Holistic Detective Agency and its sequel, The Long, Dark, Tea-Time of the Soul, which Adams himself described as "a kind of ghost-horror-detective-time-travel-romantic-comedy-epic, mainly concerned with mud, music and quantum mechanics." Even if they're not as good as the Hitchhiker series, they're still head and shoulders above anything else you're likely to come across.
If you want a taste of other science fiction comedies, it's worth taking a look at The Stainless Steel Rat by Harry Harrison, about an interstellar criminal who finds himself working for an elite law enforcement agency headed by the galaxy's greatest crook. The first book is pretty good, but there were endless sequels that each get progressively worse.
And don't forget that before he turned to fantasy, Terry Pratchett wrote Strata. It features a flat planet very like the Discworld, but this is actually taking the piss out of Ringworld. And it's by Pratchett, so you know it's going to be funny.