Philip Pullman - 'His dark materials' trilogy. Excellent, grittier and darker than you might imagine.
You read any clive barker? He writes dark, twisted fantasy stuff, often half in/half out of the real world. At his best I think he craps on koontz - try weaveworld, Books of blood, Galilee. Imajica and cabal (filmed as Nightbreed) aren't bad either. He wrote the screenplays for the Hellraiser fims.
You might also like James Herbert, the Rats books are a bit naff imo, but later books are better - Haunted, Sepulchre. He's a litle bit like King, but english.
Douglas Adams I'm assuming you're familiar with, his non-hitchhikers guide books are good, and very funny - 'Dirk Gently's Holistic Detective Agency' and 'The Long dark Teatime of the Soul' I quite often re-read when I need a reality break.
I don't know if you're into graphic novels, but there's some excellent stuff out there - Preacher (one man's mission to bring an errant and insane God to justice- you want twisted, you got twisted) and Hellblazer (v. urban, about a sorcerer geezer set in a contemporary London underworld - think James Elroy or Raymond Chandler with magic, satanism, ghosts and demons etc rather than dungeons and dragons)
Also Ray Bradbury- something wicked this way comes is an old favourite of mine.