The Book of Lost Tales was the first major work of imagination by J.R.R. Tolkien, begun in 1916-1917 when he was twenty-five years old and left incomplete several years later. It stands at the beginning of the entire conception of Middle-earth and Valinor, for the Lost Tales were the first form of the myths and legends that came to be called The Silmarillion. Embedded in English legend and English association, they are set in the narrative frame of a great westward voyage over the Ocean by a mariner named Eriel (or AElfwine) to Tol Eressea, the Lonely Isle, where Elves dwelt; from them he learned their true history, the Lost Tales of Elfinesse. In the Tales are found the earliest accounts and original ideas of Gods and Elves, Dwarves, Balrogs, and Orcs; of the Silmarils and the Two Trees of Valinor; of Nargothrond and Gondolin; of the geography and cosmography of the invented world.

The Book of Lost Tales will be published in two volumes; this first part contains the Tales of Valinor; and the second will include Beren and Luthien, Turin and the Dragon, and the only full narratives of the Necklace of the Dwarves and the Fall of Gondolin. Each tale is followed by a commentary in the form of a short essay; together with the texts of associated poems; and each volume contains extensive information on names and vocabulary in the earliest Elvish languages. Further books in this series are planned to extend the history of Middle-earth as it was refined and enlarged in later years, and will include the Long Lays of Beleriand, the Ambarkanta or Shape of the World, the Lhammas or Account of Tongues, annals, maps, and many other unpublished writings of J.R.R. Tolkien.