Blind Sight. Kathy Mallory 12

Mallory Book 12: Blind Sight is the twelfth NYPD detective Kathy Mallory novel from New York Times bestseller Carol O'Connell, master of knife-edge suspense and intricate plotting. Fans of Karin Slaughter and Stieg Larsson will be thrilled by New York's darkest cop. 'Carol O'Connell is a consummate storyteller - a unique talent who deserves to be a household name' - Val McDermid Detective Kathy Mallory.
New York's darkest. You only underestimate her once. The nun is dead. Her body lies on the mayor's lawn with three others, all killed at different times, in different places, and dumped there. There should be five - but the boy is missing. One second he was there... Jonah Quill, blind since birth, sits in a car driven by a killer and wonders where they are going. Though blind, Jonah sees more than most people do.
He's counting on this secret to save his life. Then gone. Detective Kathy Mallory is counting on herself to save his life. It takes her a while to realise that the missing-person case she is pursuing is so intimately connected to the massacre on the mayor's lawn. But she will find Jonah - she just hopes it will be in time.