Seeing any part of Hannibal is 100 x's better if you've actually read Mark Twain's works. The Becky Thatcher House is no exception. The downstairs lobby was set up as a gift shop where you can buy books. I bought a compilation of Twain's works in there relatively cheap. I remember the upstairs being set up like Becky Thatcher would have had it, but it was some time ago (about two years) since I went so my memory is a little foggy. The site isn't a must, but it is still neat if you feel like engulfing yourself into the old Tom Sawyer novel nostalgia that Hannibal puts on. I would only go if I was already in town or seeing another attraction nearby. It was either free or relatively inexpensive to see sites like this throughout the town.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.