The Lord of the Rings – 150 million
Fantasy plays a big role on this list -- and note that there are no science-fiction novels. "The Lord of the Rings" by J.R.R. Tolkien is perhaps the ultimate fantasy novel, the work that established the modern fantasy genre. It may seem like a cheat to list it as a single novel, but that's how it was written. The original publisher divided it up for logistical purposes in 1954, but for most of its history it has been sold as a single work (often in lavish boxed sets).