I have read all of Tamora Pierce's books to date, and found this one to be even better and more gripping that a number of the rest (which is really saying something as all are excellent).
The 'circle' series which this book is a part of is incredibly diverse and imaginative, and the way in which Pierce writes draws the reader in making her stories more vividly imaginable than most if not all others I have read. The view of science through the eyes of magic (or vice-versa) is truly original and spellbinding, whilst the way in which Pierce deals with the characters brings them to life; almost everything about this book makes it hard to put down.
If you have already read the rest of the circle series (The Circle of Magic quartet, The Circle Opens quartet and The Circle Reforged - The Will of the Empress) it is a real treat to visit this world again, and hear references to some very familiar people, whilst also meeting and getting to know new ones (such as Briar's student Evvy from 'The Circle Opens' book 2); the storylines overlap and come together perfectly. I was 10 when I first started to read these books, and I still really enjoy them now.
If you have not already read other Tamora Pierce books, then I would highly recommended them. If you have, then this one does not disappoint; with some series you get the feeling that they have been drawn out too long, but here Pierce really does have something new and exciting to write about.