J.R.R. Tolkien in 1916
The Book Of Lost Tales was the first major work of imagination by J.R.R. Tolkien, begun in 1916, 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 association, they are set in the narrative frame of the great westward voyage of a mariner named Eriol. His destination is Tol Eressea, the Lonely Isle where Elves dwell; from them he learns their true history, the Lost Tales of Elfinesse. The Tales include the earliest accounts 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 their invented world.

The Book Of Lost Tales is published in first two volumes of The History Of Middle-Earth, a 12-volume series of books detailing and analising Tolkien's conception of Middle-Earth legendarium. As published, The Book of Lost Tales is accompanied by extensive notes and commentaries by Tolkien's son Christopher.

The Tales are written in archaic language, so some of the words may be unfamiliar to the reader. There are also many differences with the published Silmarillion, for which these tales were precursory. If you would like to get a bigger and more detailed picture concerning these writings and how they relate to the later works, I strongly suggest you buy (or borrow in the library) the published volumes of The History Of Middle-Earth.