I should add that Fiona will be releasing a second edition of her book sometime in the near(-ish) future, so it may be worth waiting for that. I don't agree with many of her decisions there, but it is a good resource for beginners nonetheless.
Eldamo, as Celebrinor suggested, is indeed an excellent resource, though a little advanced, and could be a little confusing at first. Sadly its grammar articles on Sindarin are also not all complete yet.
Celebrinor's own website, sindarincrashcourse.neocities.org, is also a good place to look for concise summaries.
The best approach would be to combine all of the above, starting from Fiona's book and Celebrinor's website until you have some basic grasp of things, and then looking into the Eldamo grammar articles to get a deeper understanding of the context and the various debates surrounding the material.