What was J.R.R. Tolkien’s first published book of Middle-earth poetry and stories after The Hobbit?

J.R.R. Tolkien’s major published work following The Hobbit was The Lord of the Rings.

Tolkien wrote the book as a sequel to The Hobbit, but it grew into a much larger story with a deeper history, broader geography, and more complex treatment of language and power. Its central narrative follows the effort to destroy the One Ring and defeat Sauron, although this question does not ask for the Ring’s name.

The work was published in three volumes: The Fellowship of the Ring and The Two Towers in 1954, followed by The Return of the King in 1955. These volumes form one novel rather than three unrelated novels, despite the trilogy-like publication format.

Tolkien’s later posthumous publications, including The Silmarillion and Unfinished Tales, were edited from his papers by Christopher Tolkien. They are not the work that followed The Hobbit in Tolkien’s main publishing career.