Read for the Darkest London square.
Nicci French books are a counter to all those serial killer/women in peril books: there are women in peril, but the women are the protagonists, not some guy coming in to save them. More recent authors in the same vein would be Gillian Flynn and Carol Goodman. I love these sorts of books.
The Frieda Klein series is set primarily in London and one of the notable quirks of Frieda is that she walks when she can't sleep, which is often all night. There's an interesting parallel with Aaronovitch's Peter Grant series, in that both have the rivers of London running through them as a motif. Anyway Frieda spends a lot of time traveling around London, so the category brought it to mind immediately. That, and the books have a very dark side. But I love them because they also have a great warmth to them. Frieda is a therapist who ends up helping the police with their inquiries when one of her patients is murdered and over the next decade she is involved in other cases in various ways. Anyway, Frieda is a naturally solitary and intensely private one, but she is also very kind, consequently she has a large circle of friends and relations who care deeply for her, and get all up in her business. So despite having dark and brutal crimes, there is this woman on her own in a charming and snug little house, and the varied people who exasperate her with their drama but whom she remains helpful and available for. There is a balance between the two poles of alone and attached that pleases me and soothes me.
This book was a truly satisfying conclusion to the series. There's no attempt to tie up all tje loose ends, but there's quite a bit of resolution.
Highly recommended, and good for Suspence, Terrifying Women, maybe Modern Noir, Murder Most Foul, Amateur Sleuth, and arguably Slasher Stories. The first Frieda Klein novel , Blue Monday, is also the 13th Nicci French novel.