What was Tolkien’s first published book about Middle-earth after The Hobbit?

Tolkien’s first published book about Middle-earth after The Hobbit was The Lord of the Rings. It was published in three volumes between 1954 and 1955 by George Allen & Unwin.

The work began as a sequel to The Hobbit, but it grew into a much larger story involving the One Ring, Sauron, and the fate of Middle-earth. Tolkien’s publisher divided the novel into The Fellowship of the Ring, The Two Towers, and The Return of the King.

Although often called a trilogy, The Lord of the Rings is one novel with six internal books and appendices. The three-volume publication reflected printing and production circumstances rather than three entirely separate novels.

The Silmarillion contains much earlier mythology and history, but it was edited and published posthumously in 1977. Unfinished Tales and The Children of Húrin were also published after Tolkien’s death, so neither was his first published Middle-earth work after The Hobbit.