This is said to be the most accurate book on Elvish languages.
I have this one and it is short, an easy read, and reasonably inexpensive. It is said not to be too accurate on the elvish side. It is not a lingusitic work. For these reasons, I haven't used it to update my elvish words generator. I may use it to enhance some of my naming programs as the book's sections on non-elvish words are pretty well-done, and requires less accuracy -- plus I'm not so much interested in creating words with meaning, but vague meanings with fun sounds.