I was so excited to hear that David Salo, one of the linguists who worked on the Elvish for the Lord of the Rings films, was publishing a book on Sindarin grammar. Since Tolkein never wrote such a thing, it had to be reconstructed, and many attempts to do so exist on the web, with varying degrees of accuracy.
I will not say that this is definitive, because no such thing can exist. But it is useful to have so much information in one place, well organised. My favorite part is the Appendices. These include glossaries of English and Sindarin, a list of Sindarin roots (very nifty!), a glossary of names and what they mean (if you insist on giving your child a Tolkein name, please read it first!) and, best of all, a compilation of extant texts in Sindarin, always the first place you should look for grammar and ideas. I also enjoyed the section on sentence construction. There is no index, though, which is a bit of a bummer, but the table of contents is fairly well organised.
While Mr. Salo does appreciate that people write their own texts in Sindarin, this book is not for beginners, because it is a reference grammar. There are no lessons or exercises, so it should not be the first place you go to teach yourself unless you are really dedicated or have some familiarity with dead and/or fictional languages, the kind most likely to be learned from a book. For others, especially those interested in the languages as heard in the movies, I recommend a stop by Gwaith-i-Phethdain, over at [...]
For anyone who knows something about this Elvish tongue and wishes to see a comprehensive grammar, this is it. It isn't perfect, and there is plenty to squabble over, but it is a very important start.