A Killer in the Wind by Andrew Klavan
Three years ago, Dan Champion uncovered a criminal syndicate run by a kingpin known only as the Fat Woman. Champion infiltrated her world of murder, obsession, and perversity, but the case also broke him. He began hallucinating and taking drugs.
Now, Champion is a small-town detective, hoping the ghosts and hallucinations are finally behind him. Then one night he is called to examine the body of a woman who has washed ashore. To his surprise, he recognizes the woman as Samantha, someone he dreamed about during his dark period—a woman who doesn’t exist. Soon Champion is haunted by a team of expert killers who want to make sure he never finds the truth. The ghosts of the dead are all around him, and Champion has to find out who murdered them, or risk becoming one himself.