Man, the twist in that old Heinlein book 'Time for the Stars'? Wait, are we talking about the same thing? I think OP might mean 'Time and Again' by Jack Finney. That's the one with the guy who goes back to 1880s New York. The big twist isn't a single 'gotcha' moment, it's more of a gradual, unsettling realization that unravels the whole premise.
The protagonist, Si, succeeds in his time travel experiment to the past. He gets deeply involved in the lives of people there, falling in love, trying to prevent a historical crime. The twist is how the book handles causality. It leans into the idea that the past is a fixed, almost tangible place you can visit, not something easily altered. The climax isn't about changing a major event with a butterfly effect; it's about Si discovering his own personal, intimate connection to that past era, which suggests maybe he was always meant to be there. It's less a plot bomb and more a quiet, philosophical shift that makes you rethink the entire story as a kind of predestination paradox. It left me staring at the last page, not shocked, but deeply thoughtful about belonging and time.
Honestly, some readers find it anticlimactic because they want a big flashy reveal. For me, the power was in that subtle, creeping understanding that Si's journey was about finding where he fit, not rewriting history.