I think J.R.R. Tolkien is unique in having more of his work published after his death than it was while he was alive. This is due to the incredible labors of his son, Christopher, who has finally rested from his work.
The last three efforts of the younger Tolkien were not so much new material, but a consolidation of the three "Great Tales" of the Middle Earth mythos: The Children of Hurin, Beren and Luthien, and The Fall of Gondolin.
I just finished reading the middle work (Beren and Luthien) and it is excellent, though not in the conventional way.
All three of these stories go back to the very beginning of the elder Tolkien's writing career, some portions being written while he was still in the army during the Great War. As such they have a rawness that his later, more familiar work, lacks.
In the case of Beren and Luthien, the germ of the story is the romance between Tolkien and his young wife. One day, while on convalescent leave, the pair chanced to enter a woodland glade and there Edith Tolkien danced for a while in the grass. Her husband carried it with him for the rest of his life and fiction fans will know that is how Beren first became enamored of the elf-princess Luthien Tinuviel.
Decades later, their gravestones would carry the inscription of "Beren" and "Luthien" as the tale met its real-life completion.
This book explains that and other details and carries the story forward not just on its own, but through the decades that followed as its author continually revised it. Some new parts were added but other ones disappeared without any explanation.
One of these was Tivaldo, Prince of Cats, a marvelous villain and servant of Morgoth. I've seen some reviewers express disappointment that so much of the content of this book has been released before, but the passages with Tivaldo and his feline minions are worth the price of admission alone. It's humbling to see just how creative this guy was.
That said, much of the material has been published before, but never in a single, stand-alone volume, and never organized so seamlessly. This isn't like the earlier studies with extensive footnotes and variant passages. Christopher has taken great care to make the transitions between different sources flow seamlessly.
The effect is less a scholarly study and more like a gather of lore masters, each offering their tradition on how a great legend should go. One finishes, leans back from circle and takes a drink while another comes forward. "This is how our people remember the tale," he says and then offers his variation.
The upshot is that if you think this is a mere pastiche of The Silmarillion, you are very, very wrong. There are passages of intense detail here that have either never before been published or were buried in the volume the other materials provided to the public.
In fact, this book also makes a good introduction to The Silmarillion, since it carries a brief summary of the world and then gets into the action. Long-time Tolkien readers may not appreciate how much more accessible this approach is to a younger generation.
One final note: for all the 'girl power' nonsense of our age, I can't think of a modern heroine who approaches the stature and character of Luthien Tinuviel. She alone manages to humble Morgoth on his dark throne, yet she doesn't do it by acting like Turin in a skirt.
What Tolkien has done is create a uniquely feminine hero - vulnerable and loving - but also powerful enough to alter the course of history.
She doesn't toss balrogs aside or cross blades with orc-chiefs (she never even carries a weapon) but instead uses the enchantment of her voice and dance to awe and confuse her adversaries. Beren provides the muscle, but she provides the heart and inner strength and together they achieve the impossible.