Which 1954–1955 Tolkien work is divided into six books plus appendices?

The Tolkien work divided into six books plus appendices is The Lord of the Rings.

Tolkien wrote the story as a single large narrative, but its publisher issued it in three physical volumes: The Fellowship of the Ring in 1954, The Two Towers in 1954, and The Return of the King in 1955. Each volume contains parts of the internal six-book structure.

The appendices add genealogies, chronologies, calendars, writing systems, and historical notes. They make the work feel like a surviving historical record rather than a simple linear adventure. Tolkien also used the appendices to provide background that could not fit naturally into the main narrative.

The three-volume publication is sometimes called a trilogy, but the story is one novel rather than three independent novels. The Hobbit is a separate earlier book, while The Silmarillion is a posthumously edited collection of mythological and historical writings.