The Killing Doll — Ruth Rendell

In a shabby London suburb, sixteen-year-old Pup Yearman dabbles in magic. But for Pup's older sister Dolly, the magic is more than dabbling. Deformed by a facial birthmark, Dolly desperately wants to be cured, and her obsession with Pup's magic sends her on a dangerous downward spiral into confusion, madness, and possibly murder. And meanwhile, in a squalid boardinghouse not far away, a young Irishman sharpens a set of butcher knives . . .