The Book of Lost Tales was the first major work of imagination by JRR Tolkien, begun in 1916-17 when he was twenty-five years old and left incomplete weveral 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 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 Tress of Valinor; of Nargothrond and Gondolin; of the geography and cosmography of the invented world.

elrond's writeup doesn't mention that the Book of Lost Tales is a bad book. To be more precise, it is written in the overblown style of Victorian fantasy, with lots of florid verbiage and plenty of "thees" and "thous." It's interesting if you're interested in examining the development of Tolkien's thought, since it shows a first draft of Middle-Earth. However, as a work to read for itself it's a complete washout. It embodies all of the wrong ideas about fantasy that Tolkien abandoned in his later classics.