Which Tolkien book features the dragon Smaug as its principal villain?

The Hobbit features Smaug as its principal villain.

Smaug is a powerful dragon who occupies the Lonely Mountain after driving the Dwarves of Erebor from their kingdom. The Dwarves, led by Thorin Oakenshield, undertake a quest to reclaim their home and treasure. Bilbo Baggins joins them as their burglar and eventually encounters Smaug inside the mountain.

Bilbo’s conversation with Smaug reveals the dragon’s intelligence, vanity, and skill with riddles. During the encounter, Bilbo notices a vulnerable area in Smaug’s armor-like hide. He later passes that information to Bard of Lake-town, who uses the Black Arrow to kill the dragon.

Smaug is not the final source of evil in Tolkien’s wider mythology. The Hobbit also includes the Necromancer, later identified as Sauron, but Smaug is the book’s central opposing figure and its most prominent physical threat.