The Hobbit was published in 1937 as J.R.R. Tolkien’s first Middle-earth book.
The Hobbit, or There and Back Again, follows Bilbo Baggins, a comfort-loving hobbit who joins the wizard Gandalf and thirteen dwarves on a journey to recover treasure from the dragon Smaug. Tolkien wrote it as a children’s fantasy, but its setting and mythology connect it to the larger history he had been developing for years.
George Allen & Unwin published the first edition in London on 21 September 1937. The book quickly became successful, and its publisher asked Tolkien for a sequel. That request eventually led to The Lord of the Rings, which became much longer and more serious than its predecessor.
The Hobbit is sometimes described as Tolkien’s first novel, but the phrase “first Middle-earth book” is more precise for this question. Tolkien had written and circulated earlier mythological material, including versions of stories later associated with The Silmarillion, but those texts were not published as a Middle-earth book during his lifetime.