I figured the twist was gonna be the diary being unreliable, like maybe the whole affair was one-sided or something. But nope. The real gut-punch is that the wife, the one who seems like the victim in the journals? She orchestrated the whole thing. She planted the diary for her husband to find. The affair was real, but her 'heartbreak' was a performance. She wanted him to find it, to feel that guilt, to drive him into a corner so he'd finally leave her for the other woman, freeing her from a marriage she'd checked out of years before. It reframes every single entry you've just read.
You spend the whole book thinking you're witnessing this intimate betrayal, only to realize you've been watching a meticulously staged play. The twist isn't about the infidelity; it's about the manipulation behind the confession. Makes you go back and reread her 'vulnerable' moments with a whole new sense of dread.