The Tolkien book published posthumously in 1980 from writings edited by Christopher Tolkien was Unfinished Tales.
Unfinished Tales of Númenor and Middle-earth collects narratives and fragments that expand stories from Tolkien’s wider legendarium. Christopher Tolkien edited the material and supplied explanatory notes so readers could understand its variant versions and unfinished state.
The book includes accounts connected with the First Age, the Second Age, and the Third Age. Among its subjects are Númenor, Galadriel and Celeborn, the arrival of the Istari, and the history of Isildur.
Unlike a conventional finished novel, Unfinished Tales preserves incomplete passages and alternative versions. Its value lies partly in showing Tolkien’s creative process: the same characters, histories, and places could change as he continued revising his mythology.