The Lays of Beleriand, history of middle-earth, volume 3 JRR Tolkien
This was actually easy to read, and quite hard to put down. Especially when I had reached the Lay of Luthien, the poetic rhythm just kept me reading. It's also interesting from the point of relating the poem's creation to that of the prose, noting similarities, changes. CS Lewis contributes a very different style in his criticism of the lay, included at the end of the book. Sadly, most if not all are unfinished. I would have loved to read the end of the Lay of Luthien.