Murderer in the Turret?
Review of the Mysterious Press/Open Road Kindle eBook edition (2012) of the Amereon Ltd. hardcover original (1965)

This was quite an unorthodox book in the crime/mystery genre for several reasons. The culprit(s) are revealed quite early, but everyone is oblivious as to how twisted they are, except for the heroine investigator Edie. Edie takes it upon herself to not only shield the main suspect, but to also solve the crime of an attempted murder in the mansion of her relatives the Whitmans. The title room only comes into it because that is where she is hiding the suspect. The police are at first reluctant to believe her explanations, but finally the craziness will out and all is revealed.

See cover image at https://images-na.ssl-images-amazon.com/images/S/compressed.photo.goodreads.com/...
The front cover of the original 1965 Amereon Ltd. hardcover edition. Image sourced from Goodreads.

I was perhaps not as taken with The Turret Room as my GR friend Sportyrod, so I encourage you to read his 5-star review.

I discovered The Turret Room by Charlotte Armstrong from reading Christopher Fowler's excellent The Book of Forgotten Authors (2017) which I recently reviewed and rated as Five Stars. Charlotte Armstrong is the 2nd of the "Forgotten Authors" that I have investigated after first reading books by Gladys Mitchell.

Fowler describes Armstrong's style as follows:

She had abandoned a detective series to start this new style of writing, which largely avoids the whodunnit angle to portray women locked in psychological warfare with the members of their extended families and male-dominated workforces. Naturally she deals with stereotypes of the time, but the thrill comes in seeing her constrained protagonists gradually become empowered.

Trivia and Links
The Turret Room was not adapted for film, but several other Charlotte Armstrong books were. She also wrote for the Alfred Hitchcock Presents television program.

Fowler cites her book Mischief (1951) which was adapted as the 1952 film Don't Bother to Knock directed by Roy Ward Baker and starring Richard Widmark and Marilyn Monroe. Two French movies by director Claude Chabrol were based on books by Armstrong. These were Merci pour le Chocolat (2000), based on The Chocolate Cobweb (1948)) and La Rupture (1970) based on The Balloon Man (1968).

Fowler also recommends Armstrong's Night Call and Other Stories of Suspense for both the title story and The Other Shoe. ( )

I didn’t enjoy this quite as much as the other two Charlotte Armstrong novels I’ve read (‘Mischief’ and ‘A Dram of Poison’) but it was still an entertaining read. The premise is really neat (a young man who has been in a mental institution returns to the family home of his wealthy former wife to find an assault on a member of the family has taken place and he is the prime suspect) and some of the characters are great (especially the plucky Edie) but it didn’t grab me like her other two books did. It ends up feeling a little too contrived and convoluted to be truly satisfying, but it’s still gripping and the battle between Edie and her unpleasant, snobby relatives is great fun. ( )

Blurb: They said he was an insane killer. But young Edie Thompson did not believe them. She was certain that he was innocent, that his strangeness was only a mask for his terror. Was she right to hide him from the police? Was she saving a harmless youth...or setting the stage for another murder--her own?
Review: a short, tightly-focussed story of the leading family in a small town, and the unpleasantness growing in that stagnant pond. The omniscient PoV may be unsettling for a modern reader, as Armstrong shifts effortlessly from one character's thoughts to another, but this is still an absorbing fast read.