An Introduction to Elvish and to Other Tongues, Proper Names and Writing Systems of the Third Age of the Western Lands of Middle-Earth as Set Forth in the Published Writings of Professor John Ronald Reuel Tolkien
This guide to Tolkien's languages was referenced in A Secret Vice, and our library was kind enough to bring it over from North Dakota for me.
It's not a easy read. The print is tiny and in a challenging font. And it is not a book intended for dabblers. It's a for the sorts of linguists who know the International Phonetic Alphabet, love detailed discussions on where to place your tongue to achieve minute differences in vowel tones, and are fascinated by the building of languages. I would like to be that person, but am not.
The book though, is a beautiful illustration of just how passionate Tolkien was about his languages and a testament to how many other people share that passion.