Serial, by Jack Kilborn and Blake Crouch: This short (three chapters) ebook is still currently free on Amazon. From the Amazon page:
Remember the twin golden rules of hitchhiking?
# 1: Don't go hitchhiking, because the driver who picks you up could be certifiably crazy.
# 2: Don't pick up hitchhikers, because the traveler you pick up could be raving nutcase.
So what if, on some dark, isolated road, Crazy #1 offered a ride to Nutcase #2?
When two of the most twisted minds in the world of horror fiction face off, the result is SERIAL, a terrifying tale of hitchhiking gone terribly wrong. Like a deeply twisted version of an "After School Special," SERIAL is the single most persuasive public service announcement on the hazards of free car rides. Beyond a thrilling piece of horrifying suspense, SERIAL is also a groundbreaking experiment in literary collaboration. Kilborn wrote the first part. Crouch wrote the second. And they wrote the third together over email in 100-word exchanges, not aware of each other's opening section. All bets were off, and may the best psycho win.
I love murder novels (and I have no idea what that says about me). This one is the story of two serial killers, one that picks up hitchhikers, the other a hitchhiker that preys on the dopes that pick them up. It is a bit gorey, but the good ones usuallly are. Some of the book had me giggling deliciously at the characters' sheer mean-spiritedness. From what I've read, the two authors basically collaborated via email, fleshing out the story over three chapters. Very entertaining.