As has been said - nobody is another Pratchett, but others may suit your style of book.
As wella the Hich Hiker's Guide to the Galaxy, Douglas Adams wrote about Dirk Gently (Random Page on Amazon).
One author that I haven't ever got around to reading, but recommended by a friend is Tom Holt - (short extract from first book).
If you are into Sci-fi, Stanislaw Lem wrote some humorous and weird stuff as wellas "serious" SF.
Quack!
GtD.