The Silmarillion

Many people call the Silmarillion dense – they say it's too hard to read, they say it's clunky and slow-going – and it's true. But so is a lot of Lord of the Rings. Tolkien writes like a historian, in this sense, and with the attention to detail and passionate worldbuilding he put into his life's work, it's no wonder he would want to share as much of it as he possibly could, spilling out into multitudes of different volumes just to fit it all in.

I read the Silmarillion for the first time just after reading Lord of the Rings, and I devoured it. It describes the beginning, the early times in Middle-Earth, and it is absolutely fascinating. I would recommend it to anyone who wants more time in Middle-Earth.