August Books 11) Morgoth's Ring, by J.R.R. Tolkien and Christopher Tolkien
Having moved through the process of revisiting the compilation of The Lord of the Rings, the History of Middle-Earth now starts into Tolkien's later working through of his mythology. I found a lot of this material very interesting and it is a shame that more of it did not find its way into the published Silmarillion, particularly the "Annals of Aman" which brings much more detail to the early days of relations between the Valar and the Elves. Tolkien also gave a lot of thought to the question of Elvish death and immortality; there's a series of reworkings of what happened to Finwë's first wife Míriel, and also a long dialogue between Finrod and an early wise-woman, Andreth (Beren's great-aunt), about these issues. There's also the series of hints about Elvish sexuality which are nicely summarised in this classic essay, and some interesting speculation about the origin of Orcs. Binding the whole thing together is the question of Morgoth/Melkor's means and motivation; the title Morgoth's Ring is supplied by Christopher Tolkien, basically to suggest that the impact Morgoth's creative power had on Middle-Earth was similar to that of Sauron on the Rings of Power - Middle-Earth itself is therefore Morgoth's Ring in a way.
It is unusual that one could say this of the tenth book in a series of twelve, but I think I would actually recommend Morgoth's Ring rather strongly to Tolkien fans who have not tried any of the History of Middle-Earth series and are interested in giving one of the volumes a try.