Keys of Hell, by Louise Osborne [1975]

Fantastic! :D

This novel is on par with Stephen King's The Shining and Ira Levin's Rosemary's Baby, imo.

Gwen Carroll is a young journalist residing in Toronto, Ontario. She's suffered a couple of tragic losses in her life and is determined to be socially isolated. She's not bitter, but simply doesn't want to experience the pain of tragic loss again. And so she's safely esconced inside her private walls ... until the night she hears a woman screaming for help down her apartment hallway. There's a scuffle; a man is trying to forcibly take the crying child from her mother. Gwen hurries to help the woman. Gwen recognizes the woman as involved in investigating psychic activity; Gwen has recently been in the same company exploring the alleged hauntings of a nearby opulent estate, Nine Oaks. The group of would-be ghost busters is led by the handsome and charming Prof. Robert Wildfield, with whom Gwen is soon smitten.

Gwen quickly grows curious about Nonnie Richards and her dangerous situation, especially as a little girl [Laura] is involved. She is torn between fiercely maintaining her privacy and "don't get involved" sentiments versus reaching out.

It doesn't take long for Gwen to decide the right thing to do is involve herself. Maude (a now-deceased foster mother and spiritual mentor) wouldn't expect anything less from Gwen.
And so Nonnie Richards and Laura move into Gwen's apartment. The man who attempted to steal Laura away continues with stalking Nonnie's apartment and trying to break in.
Just when it seems Nonnie and Laura are off to live with a wealthy and caring uncle at the nearby secure Lion's Head estate, Gwen is unexpectedly swept back into their lives.

And then the action really starts.

The story is fast (yet smoothly) paced, characterization is strong, the descriptions unfold in the mind's eye like a movie. Gwen is a true heroine. She's brave, intelligent, conscientious, caring. Someone I'd definitely want at MY back in a dangerous situation.

A major strong point is the author leaves you guessing until the very end (with one notable exception) who the good versus bad guys are, as peripheral characters go. I had a hunch about two of them...and was proven wrong.

Keys of Hell is a true page-turner and a delight.

The cast of characters as I visualized them [drawing from quite an assortment of actors/-tresses through the years; they simply came into my mind's eye as these actors]:

Gwen Carroll: Mia Farrow [age 24]
Nonnie Richards: Liv Tyler [age 24] (especially fun visualizing her in the hair/clothing of the era; she's so beautiful)
Prof. Robert Wildfield: Harrison Ford [age 32]
Ben Potter: Fred MacMurray [age 55]
Mrs. Heaslip: Clarice Blackburn [age 50]
Nicolas Dessaix: David Selby [age 30]
Mrs. Thornby: Emmaline Henry [age 45]