The Book of Lost Tales is a collection of early stories by J. R. R. Tolkien, published as the first two volumes of Christopher Tolkien's 12-volume series The History of Middle-earth, in which he presents and analyses the manuscripts of those stories, which were the earliest form of the complex fictional myths that would eventually comprise The Silmarillion. Each of the Tales is followed by notes and a detailed commentary by Christopher Tolkien.
For publication the book was split into two volumes: The Book of Lost Tales 1 (1983) and The Book of Lost Tales 2 (1984), but this is simply an editorial division. Both volumes are separated into several "Lost Tales". More: en.wikipedia.org.