The Tolkien novel that follows Bilbo Baggins on a quest to the Lonely Mountain is The Hobbit.
Published in 1937, The Hobbit follows Bilbo as he leaves the Shire with the wizard Gandalf and a company of Dwarves led by Thorin Oakenshield. Their goal is Erebor, the Lonely Mountain, which the dragon Smaug seized from the Dwarves.
Bilbo begins as an unwilling participant but becomes essential to the expedition. He uses stealth and intelligence inside the mountain, discovers the One Ring in the tunnels beneath the Misty Mountains, and speaks with Smaug before the dragon attacks Lake-town.
The novel is sometimes grouped casually with The Lord of the Rings, but the two works differ in tone and structure. The Hobbit was written first as a children’s fantasy adventure, while its sequel developed into a much larger war-and-history narrative.