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.