The Sorrows of Young Werther by Johanna Wolfgang von Goethe
According to Wikipedia, when The Sorrows of Young Werther was published in 1774, it became the world’s first best seller. Since book publishing was, at best, only a toddler, since copyright laws were nonexistent, and since it is doubtful anyone was tracking book sells statistics, I find this claim–at least in the literal meaning...
