This is a fantastic follow-up to the previous two Sally Lockhart books, and the best of the three (I'm excluding the Tin Princess because Sally is hardly in it). Philip Pullman again shows us that he can write a childrens' book that is also perfect for adults... and in a truly adult way, as opposed to the childhood nostalgia that Harry Potter can evoke.
During the first part of the adventure, Sally falls very far indeed, losing almost everything she values, as her enemies twist the legal and social system of Victorian England against her. You end up really feeling outrage and anger on Sally's behalf, especially if you know her from the previous books.
As you read, you know there'll be a fight-back, but (if you've read his other books) you'll also know that this author doesn't always deliver a happy ending, or even characters who live to the last page. So, you're left on the edge of your seat, wondering what's going to happen...
The only criticism I'd level is that the mysterious identity of the villain is all too obvious (especially if you've read the other books), and Sally's inability to spot it is therefore frustrating. But even that doesn't take away from the excitement and tension that keeps you turning the pages to find out what happens next...