She Walks at Night by Seishi Yokomizo

Yachiyo Furugami, plagued by episodes of sleepwalking, receives a series of cryptic letters culminating in an ominous warning: 'Walk not at night'. Then a body is discovered on the grand Furugami estate, decapitated with a centuries-old samurai sword. The police are baffled, so the famous sleuth Kosuke Kindaichi is called upon to solve the mystery, but before he can do so, the killer strikes again. Can Kindaichi get to the truth before the Furugami family is destroyed by its own secrets?