The Hobbit was the first published Tolkien book to feature the mythology that later became associated with Middle-earth.
Published in 1937, The Hobbit introduces places and peoples that Tolkien later connected more fully to his wider legendarium. These include the Shire, Rivendell, Mirkwood, the Lonely Mountain, Elves, Dwarves, and the wizard Gandalf.
The book was written before Tolkien had finalized the complex history presented in The Silmarillion and before he conceived The Lord of the Rings in its eventual form. As a result, some details changed or acquired deeper explanations in later writings. The Necromancer mentioned in The Hobbit, for example, becomes identifiable as Sauron in the larger mythology.
The phrase “first published work” matters because Tolkien had written earlier versions and stories privately or in manuscript. The Hobbit is the first major published narrative in which readers encounter the evolving world that later editions and posthumous publications would expand.