Jack Vance's The Book of Dreams is the final installment of the Demon Princes series, and thus needs to be evaluated on two levels -- as a book in its own right, and as the culmination of a five-book series.
As a book, it's very good; the writing and world-building are typically excellent, and the plotting here is tighter and the pacing more suspenseful than in the previous books. This is one of the better of the Demon Prince books, and none of them have been bad.
But as the series finale, it disappoints. There were several big themes running through the series, the most interesting of which was: When you've devoted your life singlemindedly to a fixed task, what happens when you finish it? The book ends without even considering the aftermath that had been hinted at strongly throughout the series. Similarly, the chapter-intro stuff about the Institute never lead to anything big. (Is this series, perhaps, set in a larger Vanceian universe, and other novels address that theme?)
But mostly, it just didn't feel final enough. Simply by changing a few paragraphs here or there, this could have been the second, third, or fourth book in the series. There's nothing inherently wrong with that, since each book was essentially standalone, but... well, to put it into Oprah-ese, I wanted more closure.