I was digging through old Goodreads data the other day and the 2015 charts are a real snapshot of that moment. Harper Lee's 'Go Set a Watchman' was huge, obviously—the sequel/parallel novel to 'To Kill a Mockingbird' that everyone was arguing about. But the real staying power, for me, came from books that weren't necessarily the most literary. Andy Weir's 'The Martian' hit its stride that year after its 2014 release, becoming a massive crossover hit because of the movie.
Paula Hawkins' 'The Girl on the Train' was everywhere, the definitive 'girl' thriller that spawned a whole wave. And you can't talk about 2015 without 'The Nightingale' by Kristin Hannah. That book just took over historical fiction for years after. It's funny how some of these feel like yesterday, and others already have that period-piece feel, like E L James's 'Grey', which was a phenomenon but sort of faded fast.
Looking back, it was a year where movie/TV tie-ins and social-media-driven suspense really cemented their dominance in the charts.