Not a "one story in one book" kind of book. Rather, the chronological collection of various versions of the tale of Beren and Lúthien (first published in the Silmarillion, I think). Some chapters are prose, some in form of poems.
Brief versions: The mortal man Beren and the immortal elf-maid Lúthien cannot "just be together" (because her father does not approve) so they go on a quest to collect a Silmaril, the most precious gem in existence which just so happens to be in the possession of Morgoth, the most evil creature in existence. Drama ensues.
There are some parts which are dry. Christopher Tolkien goes into detail on the single fragments that he pieced together from his father's unpublished material, which is interesting in a way, but also sometimes it's not.
But then there are parts where I myself was surprised at how captivating a poem spanning 20+ pages can be. The tale itself is beautiful and dragged me into the realm of middle-earth again.