As the next-to-last book, this one is building to the final confrontation. It is fairly convoluted, as most every character that has survived the first four books is showing up, along with new ones, plus we have the added confusion of time travel. It is still enjoyable, but less fascinating than the first two... even, perhaps the second two. It has a bit of an "oh shit, we only have two books left" feel while still feeling that Scott knows where he is going... if that makes sense?
If you have read the series so far, you will obviously want to read this one. The problem is remembering everyone and everything that has happened thus far. The major players have changed little, and are the core of the narrative, but they get a bit washed out by all the other characters. Dr. John Dee and Virginia Dare are the focus, while there is a nice bit of the growing friendship between Machiavelli and Billy the Kid. I get the feeling Machiavelli is going to go Severus Snape on us, but it is hard to tell.
As for those of you who have not started the series... Well. If you like YA literature, and like fantasy, and do not mind reading six books to get a conclusion, you should give it a try. If you don't care to invest that kind of time in a series, don't bother. The first two books might suck you in, but they are unsatisfying individually. There is always the cliffhanger, and the reminder that "nothing is over yet" looms large. You can see my previous mentions of the series here.
By the way, if Daniel Handler is reading this I will obviously make an exception for a new Lemony Snicket series. ;-)
As I near the end of the book it becomes more compelling. They do spend a lot of time covering older material and reacquainting you with secondary characters, but at the half-way point it turns around. Unfortunately, the excitement seems to be just building to the final book, and a huge cliffhanger seems assured.