Harris, C. S. Sebastian St. Cyr mystery series.

"On a sunny day in July 1815, thirty-eight-year-old Philippa, Lady McKinsey, takes her sixteen-year-old daughter, Emma, and her young niece and nephew, fifteen-year-old Arabella and thirteen-year-old Percy, on an outing to Richmond Park. But when Arabella and Percy go off to pick flowers, tragedy strikes. Shots echo across the park. Two young gentlemen investigate and find Lady McKinsey and her daughter dead. As the men gaze in horror at the strangely posed bodies of the victims, the other two children come up laughing, their arms full of blossoms. Arabella opens her mouth to scream, but there is no sound. Sir Henry Lovejoy, Bow Street magistrate and good friend of Sebastian St. Cyr, Viscount Devlin, finds himself reliving a nightmare. Fourteen years before, Lovejoy's own wife and daughter were murdered in Richmond Park, their bodies posed in the same bizarre fashion. Lovejoy himself had been instrumental in the arrest of the ex-soldier later found guilty of the killings, and he'd watched the man hang with grim satisfaction. Now he must turn to Sebastian for help as he confronts the very real possibility that he helped send an innocent man to the gallows, and that the monster responsible for the deaths of his wife and daughter is still at large--and has killed again"--