THE ALCHEMYST (The Secrets of the Immortal Nicholas Flamel) by Mixhael Scott

Sophie and Josh, normal teenage twins, are spending the summer in California with their aunt while their archeologist parents are off on another dig in the desert. They have each found a job to keep them busy, Josh in a bookstore and Sophie in a coffee shop across the street. One day a couple of men in sunglasses show up at the bookstore and nothing is ever the same again. Sophie and Josh are drawn into an adventure with 600 year old people all of whom look under forty and must help them save a book, which in the wrong hands, could destroy the world. During their adventures, they discover some magical things about themselves as well.