Following J.R.R. Tolkien's death, his son Christopher Tolkien began combing through Tolkien's papers to provide the world with posthumous works much in demand. Among the material the Christopher published is the twelve-volume series The History of Middle-earth. Volume Five The Lost Road and Other Writings is among the most important of the series. In it, we find versions of the stories that serve as the backdrop for The Hobbit and especially The Lord of the Rings. Furthermore, the versions in Volume Five were written just before the time of the writing of The Lord of the Rings. Consequently, they give us the clearest picture of Tolkien's legendarium as it bears on understanding The Lord of the Rings.
The first part of the work details the history of Numenor and its fall. Numenor was an island created for the race of men who fought with their half-kin the elves in the epic battle against Morgoth. Ultimately, the Numenoreans were seduced by Saron into waging war against the Valar, the gods who inhabited the forbidden Western land of Valinor. Upon the defeat of the Numenoreans, the island of Numenor was submerged into the ocean, with only a remnant of the race (loyal to the gods) escaping to Middle-earth. With the destruction of Numenor, the Valar reshaped the planet -- Arda -- such that it was now impossible for mortals to travel "the road" to the forbidden shores of Valinor, forever separating the men Middle-earth from alinor; hence, the story of "the lost road."
The Lost Road itself was an attempt by Tolkien to write a time travel story in which the travelers found their way back to Numenor through the vehicle of dreaming. The Lost Road was never completed, though Tolkien again attempted the story in a later work known as The Notion Club Papers. The Notion Club Papers can be found in Volume Nine of The History of Middle-earth -- Sauron Defeated.
Time travel as conceived in The Lost Road and The Notion Club Papers bears an interesting relationship to the work that Tolkien was engaged in as a philologist. Reconstructing dead, prehistoric languages from the remnants of descendant historical languages and thereby recreating the prehistoric culture is at least as much an art as a science. It inevitably involves creativity and imagination which are most at liberty in our dreams. How much Tolkien saw his work as a philologist as traveling through time is an open question, but The Lost Road is strong evidence that this is how he conceived of it.
Part two of Volume Five reaches even further back in the history of Arda, providing a description of its creation, annals of the world before the fall of Morgoth, and a version of the Quenta Silmarillion which tells the history of elves from their origins to the fall of their arch enemy, Morgoth. It also includes a version of The Llammas, a treatise on the history of the languages of the people of Arda.
Much of the material in Volume Five appears in earlier published work by Tolkien, particularly The Silmarillion. After each section by Tolkien, Christorpher makes an heroic effort to describe how the present version differs from other versions, but the level of detail is too great for the casual reader to appreciate the distinctions. Setting the texts side by side and using Christopher's notes as a guide might yield valuable insight into the transformation of Tolkien's creation, but in the end, it would probably only be of interest to the most dedicated Tolkien scholar. Nonetheless, Tolkien's narative, given to us in The Lost Road and Other Writings will reward anyone who appreciates Tolkien's work.