Loved this book - it's very, very clever. It's a great middle-to-upper primary read. The story is about four highly gifted children who are selected to help defeat an evil mastermind. The great villain can only be defeated by children and it is they that must face all kinds of difficulties, puzzles and mysteries to save the world. One of the consistent themes in the book is creative problem solving and the reader is allowed time to wonder how they are going to get out of each puzzling situation and solve the riddles as they come along. As we read the book aloud, we stopped and discussed the various problems and tried to guess the solution. Good fun.
But the real highlight of the book is the deeper themes it addresses: truth, courage, the need to belong, self-sacrifice and bravery. In particular, the idea that we need to think through the messages that are fed to us (through the media, television, education etc.) and stand up for what is true and good forms a major part of the plot itself. Although this would be a fun book for an upper primary kid to read on their own, I think it is much better as a read-aloud book that you share and discuss as you go along. That way you can hopefully launch into some really interesting discussions based on the dilemmas faced by the main characters.
A little warning though - for very sensitive children, the kidnapping and constant suspense and danger might be a bit much. Under 8 - use caution depending on your child. Over 8 should be fine.
I give it five stars and I'll definitely be chasing down the sequels.