In which 1937 book did J.R.R. Tolkien introduce Bilbo Baggins?

J.R.R. Tolkien introduced Bilbo Baggins in The Hobbit, published in 1937.

The book began partly as a story Tolkien told to his children, but it developed into a complete adventure about Bilbo, a comfort-loving hobbit recruited by Gandalf and the Dwarf king Thorin Oakenshield. Their company travels to the Lonely Mountain to reclaim treasure from the dragon Smaug.

The Hobbit also introduces the mysterious ring Bilbo finds in the Misty Mountains. Tolkien later connected that ring to the larger mythology of The Lord of the Rings, which changed the apparent scale and significance of the earlier story. The Hobbit is sometimes described as the first Middle-earth novel, although Tolkien’s mythology of Arda had been developing in stories and languages for decades before its publication.