Beren and Lúthien just collects different versions of the story and presents them in a way that shows the evolution of the tale from the earliest drafts to the latest. In the words of Christopher Tolkien in the preface,
this book does not offer a single page of original and unpublished work.
The book I sentis just a nice way of having different versions together, with new commentary and explanations from Christopher, and the great illustrations from Alan Lee.
The way it was marketed is certainly confusing, and led many people to expect a complete story in more of a novel form.