You can, but only if you're interested in how Tolkien developed his world. The Book of Lost Tales
(both of them) contain some of the earliest versions of the stories that eventually were published by Tolkien's son in The Silmarillion
. They're the first two books of a twelve-book series, The History of Middle-earth
, that detail the development of Middle-earth from Tolkien's earliest ideas around World War I up to assorted writings from the last years of his life.
They're more like a non-fictional biography of Middle-earth than they are actual novels or anything like that.