If you're at all a fan of the hammer horror film, "Die! Die! My Darling" aka "Fanatic" (1965) then this is worth your time to get into the character's head space and motives a bit more (plus it's free on open library), but otherwise it has its flaws that held it back from its potential (and more than a nomination for an Edgar award). Pat's perspective chapters are the best because they are the most climactic, and it's easy to sympathize with and root for Glentower in his fight between emotional hunches and reasoning with very little clues to go off of. And then you've got Mrs. Trefoile, a pre-Misery, psycho-biddy, would-be mother-in-law from hell, religious zealot and closet sadist to boot. What a set-up! But where it doesn't quite cut out is in putting so much of the narrative focus on the fateful threads of communications, misfires, and happenstances that set the stage for Pat's rescue instead of the much more interesting predicament she's in and the dissolving psychological morale for everyone involved. And when the rescue finally does occur, the showdown between Glentower and Trefoile is very matter of fact with no epilogue for Pat to make up for the tortures she suffered.
Some notable differences between the book and the film:
-The book is more violent, and Mrs. Trefoile's and Anna's blood thirst and unconscious lust for power and dominion over others under the guise of religious conviction is explicitly defined.
-Unlike the film, Henry/Harry is not a malicious creep in the book. Criminal on the lam or not, he never tries to harass Pat and in fact is the only one in the household that wants her to remain unharmed and set free. He is also the only one who seems to see the writing on the wall that Mrs. Trefoile is batshit and can't be trusted to run her own household or make decisions.
-Mrs. Trefoile's relationship to her son is less of a motive for her in the book than it is in the film. Pat is only able to thinly surmise what his childhood must have been or how close he ever was to his mother. It becomes clear pretty quickly that her devotion to her son is just another excuse to overpower Pat.
-In the film, from what I remember, it is heavily implied that Mrs. Trefoile was once a semi-successful actress or singer before she retired and turned into the religious mess. In the book, the village people say she was always a fanatic, and came from old money.
-In the film, from what I recall, Glentower has a much quicker epiphany as to Pat's whereabouts. In the book, it takes like twenty flashing red signs for him to put the pieces together. This being the case, there are a lot more throwaway side characters like Anna's niece, Gloria, to act as clues for him.