Five stars, absolutely. This is such a fun book– like Charlie and the Chocolate Factory, 1984, and A Wrinkle in Time rolled into one.
It begins with an orphan who is a genius. While reading the paper with his tutor at the orphanage, he sees an ad addressed to children who who would like special opportunities. He sits for the exams and becomes part of the Mysterious Benedict Society, whose purpose is to save the world.
The book is full of puzzles and quirky characters and suspense– I could hardly put it down.