This book is packed with enough ideas to pad out an entire career for many writers. From speculations on the nature of intelligence and technology to wonderfully inventive aliens, and with a good bit of space opera mixed in, this book has it all. It reminds me of nothing so much as Startide Rising, though the books are actually quite different in tone.
This is a prequel to A Fire Upon the Deep, despite which it is an excellent book. I'm not normally a big fan of prequels, because I dislike already knowing the ending to the story. But here, Vinge presents us with a story to which we don't already know the ending by telling us the tale of Pham Nuwen, whose memories of his past in Fire were sketchy and of questionable accuracy. He also sets the story in a different part of the universe, where even the laws of physics work differently. In these ways, Vinge avoids all the customary traps into which prequels are wont to fall. And while he's busy performing that bit of deft footwork, he manages to throw in yet another race of interesting aliens, some nifty stylistic tricks, an involving and interesting plot, lots of neat sense-of-wonder stuff, and some interesting characters. This book is certainly the equal of the excellent Fire, and possibly its better -- a rare feat indeed for a prequel.
The easiest way to describe Illusion is to call it "A Tale of Two Cities with magic." But while that description succeeds in getting across the general tone of Volsky's novel, it's not entirely accurate. Yes, Ilusion is set in a fantasy version of the French Revolution; yes, it's a grand sweeping work set over a period of many years; nevertheless, it is still very much its own book. And what a book it is: Illusion, more than any other book I've read, captures the feeling of what it must be like at the crux of a revolution.