Thirteen Steps Down by Ruth Rendell
A classic Rendellian loner, Mix Cellini is superstitious about the number 13. Living in a decaying house in Notting Hill, Mix is obsessed with 10 Rillington Place, where the notorious John Christie committed a series of foul murders. He is also infatuated with a beautiful model who lives nearby - a woman who would not look at him twice.
Mix's landlady, Gwendolen Chawcer is equally reclusive - living her life through her library of books.
Both landlady and lodger are caught up in their own psychologically twisted parallel worlds. But when reality intrudes into Mix's life, a long pent-up violence explodes.