This book is fundamentally unrateable. It’s an academic chronicle of Tolkien’s approach to the story of Beren and Luthien so it is both the narrative and about the narrative. The story sections are good, if sometimes frustrating in brevity, Christopher’s glosses are either far too short to provide meaningful context or repetitive.
And yet it’s Tolkien. And when the stuff gets out of the way and the epic poetry fades through...well, if that is your thing, this will most certainly be your thing.