The Mysterious Benedict Society and the Prisoner's Dilemma by Trenton Lee Stewart is the third book about a group of four extraordinarily gifted and adventurous children who call themselves the Mysterious Benedict Society.
Since this is the third book in the series, I won't say much here about the plot. I've enjoyed all three books, and while the first will always be my favorite, I think Prisoner's Dilemma is the second best. I enjoyed that they were back home at Mr. Benedict's mansion for most of this one, and the action built up nicely.
The MBS books in general are fabulous - they have it all: great characters, illustrations that are a bit Gorey-esque, a formidable group of bad guys, logic puzzles, brain teasers, and pie.
This series is outstanding, and is a perfect gift for kids (boys or girls) from about 5th grade through 9th. It's also a series that adults who enjoy YA & kid's novels will love too; my mom and many of my friends have read and loved them.
For anyone wanting to give this series a try, I recommend starting with the first book, The Mysterious Benedict Society, which is available in paperback from The Book Depository for only $3.49.