Trying to define popularity by total readers is a real puzzle. Sales figures get thrown around a lot, but they’re often muddled by how we define a ‘copy sold’ over centuries. I’d argue the holy trinity is probably 'Don Quixote', 'A Tale of Two Cities', and 'The Little Prince'. The first is foundational for the modern novel, the second has been a school staple in the English-speaking world for generations, and the latter’s translations are insane. It’s not about who’s reading them right this second, but their cumulative cultural footprint.
Yet, those feel like ‘assigned’ popularity. For organic, sustained global engagement, you have to look at 20th-century series that became multimedia empires. 'Harry Potter' and 'The Lord of the Rings' are in a league of their own. They created readers. I’ve met people who learned English just to read them in the original. That kind of reach, where a story becomes a gateway to language and community, is a different metric of popularity altogether. Modern Chinese web novels like 'The Three-Body Problem' are achieving similar cross-cultural status now, but their total reader numbers are still catching up.
Still, nothing will ever touch the sheer volume of a book that was, for a time, the only book a vast portion of the population was allowed or encouraged to read. So, by the numbers, the Bible has to be in the conversation, though it complicates the definition of a ‘novel’. It ends up being a debate about what we’re actually measuring.