Rendell Ruth. Thirteen Steps Down

Psychological thriller novel by Ruth Rendell.
Mix Cellini is a lonely, maladjusted young man who works for a company that repairs exercise equipment, and lives in the upstairs apartment of an old Victorian house on Notting Hill. While his reclusive landlady, Gwendoline Chawcer, spends her time reading and pondering lost loves, Mix grows dangerously obsessed with serial killer John Christie and a local model, Nerissa Nash, despite the fact that she hardly even acknowledges his existence.