Book excerpt: Faith and Summer Castle are the ultimate crime fighting duo. And they're not even old enough to drive. Faith has the super smarts and Summer brings the super brawn. When their Uncle gets kidnapped by the evil Agent Dark the Castle Sisters find themselves facing their greatest challenge yet. The forces of darkness are gathering and the battle between good and evil has reached a pivotal turning point. Agent Dark has stolen a secret formula that could give him the ultimate control. With the sisters' Uncle out of the way, no one stands between Agent Dark and the end of everything Not only does the fate of the world now hang in the balance, but the sisters must pull off what will almost certainly be an impossible rescue.