Silmarillion

Then this is the book for you. I would recommend first reading both the Hobbit and the Lord of the Rings before tackling the Silmarillion; it is a fairly dry read since it is written more or less as a synopsis on the history of Arda from the very beginning when Eru, the one God, created the lesser gods who then created the world, and up to the beginning of the Lord of the Rings. It is a lot of history to get through in such a relatively short book, so not much detail is given to each event and like our real world history there are a lot of people and a lot of names to keep straight. It is not an easy read. But for those who want to understand what led up to the events in Lord of the Rings, it is the perfect place to go. This book will answer questions like why Elrond is so respected among elves, how it is even possible for elves to choose whether they should be mortal or not, why Aragorn being of Numenorean descent matters, where Sauron came from, where Gandalf and Sauruman came from and what led them to act the way they did and make the choices they did. If the answers to these questions interests you and you are willing to dig into the history, then please, get this book and read it slowly - maybe even take some notes while you're at it; it's worth it!