The Third Knock

Detective Mara Vance knocks three times before entering any room. Three times on every door. Three times on her own apartment frame. The habit kept her alive once. Now it's the only reason she's still breathing. A house. Three previous owners. Three "accidental" deaths. All on the eleventh night. All after three knocks. Mara has seven days to find the pattern before the house knocks back. No one believes a woman who knocks at nothing.