I Am the Angel — Michael Stuart

When he's killed at a young age in a car accident, the unnamed narrator enters the spirit world and is surprised to learn that the angels want him to join their ranks. Upon arriving in Heaven, he discovers an idealized world of comfort, but the euphoria doesn't last, as he's forced to return to the rigors of school in an angel-training course. Soon he discovers that the many disappointments of his life have left him with a bitter disdain for all of mankind, and that an angel's existence might not be for him.
Then he befriends Marley, a fellow angel-trainee. After a life of tragedy and addiction ending in overdose, she doesn't feel worthy of receiving the halo herself. But amidst a surreal world of asexual attraction, guardian spirits who curse like truck drivers, and a supreme being who might possibly enjoy waiting diner tables, the two start to help each other overcome their metaphysical and personal distress. It seems in death, you might learn there's more to you than you ever knew in life.