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

Tolkien's Middle-earth mythology — built over 50 years from The Hobbit (1937) through The Lord of the Rings (1954-55) and the posthumous Silmarillion — is the most complete invented world in literary history and the foundation of nearly all subsequent secondary-world fantasy. 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.