If your child loves challenging puzzles, he/she'll love this book. "Witness to a dramatic kidnap, Mel is plunged into a twilight world of gangsters and secret agents. A coded message leads her down into the sewers and subways, and up to the glitzy heights of Megabuck Towers in a bid to foil a sinister plot to take over the world...Codename Quicksilver is a thrilling adventure story packed with fiendish codes and puzzles which must be solved to unravel the plot. Most of the puzzles are pretty difficult, so don't be surprised if you get stuck."
The book contains both clues and answers.