The Lord of the Rings — J.R.R. Tolkien

The Lord of the Rings is the most complete invented world in literary history, built over 50 years from The Hobbit (1937) through The Lord of the Rings (1954-55) and the posthumous Silmarillion. It has sold over 150 million copies, and its adapted films grossed $5.9 billion. No fantasy series approaches its depth of linguistic, mythological, and cultural invention — even Harry Potter, at 600 million copies, does not rival Tolkien's world-building density or influence on the genre.