Sidereal is a hardcover that presents a new "faction" of Exalted - the Sidereal, the weavers of Fate and the manipulators of Destiny. It's an interesting concept, but something that requires yoiu be very "in" to the particular setting. You alternate between fulfilling the objectives of your superiors and the gods you serve (the Maidens of the Chosen), some adventuring on your own, and working in the Celestial Heaven (of the Chinese variety - a big bureaucracy) and negotiating with the gods. And running your own schemes and plans in the real world when you get a chance in your busy schedule.
Which is part of why I'm not sure you'd want to run a Sidereal campaign. They're easily forgotten (by other Exalteds isn't quite as clear), work behind the scenes, compared to other Exalteds their numbers are small, and if they die it takes a while for them to come back...although they do come back, unlike Dragon-Blooded. They touch on this briefly in the back, but it's still a bit awkward.
The background and setting stuff is useful, but then we get into the game mechanics. The Charms are okay - powerful, somewhat oblique (what does stealing someone's name really _do_ again, other then make it difficult for them to be invited to parties? :) ), and a bit...quirky. Larceny charms steal just about anything except...well, real stuff for the most part.
But the astrology rules - oi vei! Complicated and not a lot of major impact for the effort. The rules aren't very linear, and as seems to be the current trend in a lot of RPGs, no examples are given.
There is a random "assignment" chart for missions the Sidereals go on to change fate, often times without knowing what the long-term implications of what they're accomplishing will be. There's a kind of "butterfly effect" to their missions ("Make sure X marries Y"), at least at the lower levels where presumably such campaigns are played. There's kind of a Quantum Leap feel to this part.
Also, for the first time the chapter fiction took a bit of a dip. The one with the demon-hunting Sidereal is pretty bad, and the one with the hunted Solar who refers to his Charm names in quotes (ugh) is rather awkward.
Overall this book is good quality, but more as a sourcebook that provides some deep insights into the setting. You could run a Sidereal campaign (and I'm sure there are folks out there that do), but it's a bit limited.