The Cradle in the Grave by Sophie Hannah

From Library Journal: "Within the span of a week, a crazed killer has gripped London by attacking mothers who have been convicted—and acquitted—in a sensationalist rash of crib death cases. One of the victims is known for her heartrending memoir and is also the centerpiece of an in-progress TV documentary. A puzzle card is left on her body; the same card is mailed to the TV producers. Hannah's frenetically paced psychological thriller is a searing indictment of our judicial system and the media's role in fueling collective hysteria. Known for combining her police procedurals with woman-in-peril suspense novels, she again succeeds in scaring our shoes off."