From Pratchett

[The Enigmatic Dr X]

Just bought "And Another Thing", the new Hitchiker's book by Eoin Colfer. No idea if it's any good.

I really liked Tom Holt's Portable Door and its couple of sequels (You Don't Have to Be Evil to Work Here But It Helps, and Earth, Air, Fire and Custard).

I'm also assuming you've read Good Omens. Everyone really should.

Another couple of similar books, with a twisted reality but played less for gags are:

The Raw Shark Texts, Steven Hall. I really liked this. It's indescribable.

The Pilo Family Circus, Will Elliot. Killer Klowns from Hell.

[Jared Katooie]

Raymond Chandler.

[Albion]

Harry Harrison.
I started reading his books after The Stainless Steel Rat was in 2000AD.

[O Lucky Stevie!]

Stephen Donaldson's always good for a chuckle.

[Cactus]

You should try some Jasper Fforde.

[Paul faplad Finch]

I think Fforde tried to get a little bit too clever for his own good in the later Thursday Next books but the first couple, and the Nursery Crime novels are works of genius.