The Silmarillion

Designed to take fans of The Hobbit and The Lord of the Rings deeper into the myths and legends of Middle-earth

This is the Book of the Elves, if you will, of their glory days and terrible doom. Fel oaths are sworn, battles are fought, lamentations are sung, all for the three Silmarils wrought by Fëanor. It doesn't end well, but my goodness, there is so much beauty in this sorrow, too, not least the tale of Beren and Lúthien.
If you love The Lord of the Rings, and I mean the books, not the movies (great though they were in their own right), you will love The Silmarillion. It might take you a few tries to get started because Tolkien's style is grand and rather biblical, but once the Elves appear, and later the Men, you won't be able to stop reading. Eärendil, Gil-Galad, Elbereth - all those names sung by the Elves all through LotR, they come to life here. A tale full of heroism, wisdom and sadness.