Any popular book today is by definition a bestseller, as most books are being sold on the book market. Authors don’t become well-known unless their books get into as many hands as possible. And that can’t happen unless books get sold. The popularity of a writer is a function of the market. There are many books and many authors out there who fail to achieve popularity because of the market imperative. The books may be good, they may capture the hearts and minds of readers, but the publishers are not always well-known, the business models are not always great and the books fail to reach their intended audience. Social media seems to be disrupting this model by having more people promote lesser known titles and authors, but the long tail of the book market remains long and unknown.
This market model of book/author popularity has never been the only model. It has became prevalent with the development of the book market since the early modern period. Before that, authors became famous and books became popular in different ways. In the Roman Empire, Virgil’s epic poem The Aeneid became the most popular book, achieving even school curriculum status (it stayed in the classroom for almost 2000 years). The Aeneid didn’t become a ‘bestseller’ because of a business model, the Roman book market (which wasn’t bad by ancient or modern standards) or through shrewd publishing. It was propelled to the top due to imperial patronage, having probably been commissioned by the Emperor Augustus himself. What the emperor commands, the book trade provides, and so the Aeneid permeated the Roman literate circles (which weren’t few).
With the decline of literacy following the collapse of Roman imperial structures in the West, the book-author popularity model came to focus on reader response. Unlike our age when books are generally instantly available for purchase, the medieval book model was order- and commission-dependent, not unlike modern custom-made furniture. Books got copied because their readers wanted them copied. It wouldn’t make much sense to lavish human and non-human capital on producing a book that may not get read, let alone sold. So the readers were driving the market. The French medieval poet Chrétien de Troyes, for instance, whose chivalric romances saw huge popularity throughout the Middle Ages (and inspired others to write similar ones), became a bestselling author because his readers were pushing for more books to be copied. It was out of his hands, and without publishers to control production, marketing and distribution, his books stayed in the public domain.