The debate about Ruth Ware's most psychological thrillers always circles back to 'The Turn of the Key' and 'The Death of Mrs. Westaway' for me, but I'd argue the ranking depends entirely on what kind of mental spiral you're after.
'In a Dark, Dark Wood' gets points for pure, claustrophobic paranoia—that feeling of being trapped with people you can't trust, where the setting itself becomes a character messing with the protagonist's head. It's less about a grand, external plot and more about the slow unraveling of memory and social anxiety under pressure. The psychological tension comes from the relentless second-guessing, both for the character and the reader.
But 'The Turn of the Key' uses the modern fears of technology and surveillance to create a different kind of dread. It's a haunted house story where the house is a 'smart home,' and the unreliability isn't just in the narrator's memory but in the very devices meant to provide security. The ambiguity of Rowan's situation, paired with the formal device of her writing letters from prison, forces you to constantly reassess her guilt, sanity, and perception. That lingering doubt about what's real and what's manufactured by her environment or her own mind sticks with you longer, I think, than the solution in 'In a Dark, Dark Wood.'
So for a ranking focused purely on sustained psychological unease, I'd slot 'The Turn of the Key' first, followed closely by 'The Death of Mrs. Westaway' for its masterful exploration of identity theft and the corrosive weight of a lie, then 'In a Dark, Dark Wood.' 'The Woman in Cabin 10' feels more like a straight-up mystery thriller—tense, yes, but the psychology is more situational panic than deep, character-driven fracture.