To expand: The Lord of the Rings is included in this quiz because it was intended to be (and was, according to Tolkien - despite being split) published as a single book. It was, yes, divided into 3 parts (The Fellowship of the Ring, The Two Towers and the Return of the King), and then a further 3 "books" within those books WITH chapters in them. Despite it seeming that the LOTR should've been published into 9 parts, it is generally considered to be one epic-novel. Think of the Fellowship/Towers/Return as dividers in one huge book, with further dividers within them. I tried to explain this briefly in the caveats, but for those are discontented, this is the reason why the LOTR is, on this list, the best-selling book of all time.
Nooo... I didn't include A Tale of Two Cities or the Bible because they're not among the best-sellig books of all time that fit this criteria. No doubt, the Bible has sold millions of copies and A Tale of Two Cities is an excellent book, but there is either no reliable account of the numbers sold, or quite simply they haven't sold equal to or more than 35 million copies.