J.R.R. Tolkien kept notes on various aspects of his Lord of the Rings books for many years. In 1977, his son, Christopher, published some of these as The Silmarillion. It is important to know that these notes were in a rough form. The book, while invaluable to the back story of Middle Earth, is very dense and more suited to studying than reading, since the preponderance of names (most of the characters seem to have three or four), places and small details make this difficult going for the casual reader.
This said, my husband has this practically memorized. If your library is frequented by Tolkien fanatics, you may need a copy of this, although the length of this makes it something that students may want to have for their own. My 7th grade son started this over the weekend and did not find it as impenetrable as I did. I have been working on this since August, and seriously had trouble staying awake while reading this.
It has come to my attention that there is also The War of the Jewels : the Later Silmarillion, Part II, The Legends of Beleriand. While the very thought of this makes me want to weep openly, it would be a lovely Christmas present for the person who adores The Silmarillion.