|J.R.R. Tolkien in 1916|
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.