No Man - Book 1, #1

He was left to die in the steaming jungles of Vietnam by his commanding officer and the rest of his battalion back in 1975; they should not have done that. Forty years later... Homicide Detective Iris Williams and her partner Detective Annette Toni are assigned to a case where the victims are left most bizarrely. The killer's hiding place is in the abandoned subway tunnels under New York City. Detective Iris Williams must face her fears and venture into the rat-infested tunnels to apprehend the most diabolical serial killer she'd ever encountered, a serial killer that will change her life forever.
The media is calling him, The Piggyback Killer...