Beast in the Shadows by Edogawa Rampo

A mystery writer turns detective to protect the woman he loves. But is he hunter or hunted?

The chance meeting between a crime novelist and a married woman blossoms into friendship. When she confides to him that she has been receiving threatening and sadistic letters from an ex-lover, who says he is watching her in the shadows, he knows he must help her. But the trail unexpectedly leads to another writer, Oe Shundei, the mysterious and secretive author of works of grotesque violence. Suddenly nothing is as it seems, and nobody is safe.