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.