I would also suggest Robert Asprin...his stuff is not really like Rankin's, but it is genuinely good fun. Maybe start with the Myth or Phule series of books. Enjoy.
The Leaky Establishment.
I put it right up there with the best of Adams, Pratchett and Rankin. In fact, it would make my top three funniest novels ever with Adams's HHGG and TP & NG's Good Omens.