This review will contain spoilers!
In many sentences:
There isn't much that can be said about Rise of the Evening Star that I didn't cover in my critique of Fablehaven. It is an exciting and fun book to read. In a way, it is a palette cleanser between more serious or dense works (or one's that are simply not fun to read). It moves quickly and is extremely imaginative - a point which I don't think I brought enough attention to when discussing Fablehaven. The level of creativity in each of these books should not be overlooked - every page seems to introduce some new and exciting piece of fairy lore without feeling forced or out of place. While it may not have the depth of Harry Potter, it does have a much more rich menagerie of creatures.
Another thing worth noting is that, a few examples aside, this book does not suffer from the scenario of the author introducing new items, characters, or rules for the sake of explaining some new plot element. Instead, most of the major items of concern in this book are consistent with the first. If nothing else, it shows that Mull had the series (at least up to this point) in mind from the beginning.
As far as the major plot twist goes - that Sphinx may be a double agent - I think this is certainly the case. From the beginning, they often say that the Society operates with "infinite subtlety and patience". Upon reading this for the first time I thought "If this is true, then the Sphinx must be the ultimate enemy, for who else could such a phrase apply?" (The Sphinx is hundreds of years old).
Two final thoughts: first, I would recommend this as strongly as the first. Second, when the character Vanessa was introduced I mentioned it to my wife Vanessa and she said "Is she bad? Characters named Vanessa are always bad." She was, it turns out, right after all.
Brandon Mull the Last Line of in Rise of the Evening Star