It's the story of the story of Beren & Luthien. If you've already read the Silmarillion, then you won't really find many new tidbits on Tolkien's mythos. The value of the book is in showing the evolution of the story's concepts through the years. It's very scholarly in its execution, but I found it was not quite as fun to read as the Children of Hurin.

Much of it was in verse, and I'm not a big fan of extended verse, so you may find this volume more to your taste than I did. It was interesting to see the evolution of Sauron's backtory, however, from the supplanted king of cats to the lord of werewolves, to the necromancer Thu.