I enjoy Kurt Vonneghut even more than Heller. Scathing, twisted, bitter humor. Start with "Slaughterhouse Five," then maybe "The Sirens of Titan." You will probably want to read everything he's written.
Tom Robbins also has brilliantly wicked stories, such as "Fierce Invalids Home from Hot Climates" and "Half Asleep in Frog Pajamas."
Charles Bukowski is also classic. "Ham on Rye" and "Post Office" are good ones.
You may or may not like Salmon Rushdie. His novels are a little darker, but still good. "Satanic Verses" and "Fury" are my favorites.
Haruki Murakami is also an absurdist genius. Books are witty but, again, pretty dark. "The Wind up Bird Chronicles" and "Hard Boiled Wonderland and the End of the World."
If you want to go even deeper into the abyss, try Kafka ("The Metamorphosis," "The Trial") or Camus ("The Stranger," "The Fall," etc).
