The Book of Lost Tales was the first major work of imagination by J.R.R. Tolkien, begun in 1916-17 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 these tales were the first form of the myths and legends that came to be called The Silmarillion.

Embedded in English legend, they are set in the narrative frame of a great westward voyage over the Ocean by a mariner named Eriol (or AElfwine) to Tol Eressea, the Lonely Isle, where elves dwelt; from him they learned their true history, the Lost Tales of Elfinesse. In these 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 cosmology of Middle-earth.

Volume One contains the tales of The Music of the Ainur, The Building of valinor, The Chaining of Melko, The coming of the Elves and The Flight of the Noldoli, among others. Each tale is followed by a short essay by Christopher Tolkien, the author's son and literary executor.