The Alchemyst by Michael Scott

15-year-old twins, Sophie and Josh are working at summer jobs in San Francisco when they are inadvertently pulled into a struggle between alchemists Nicholas Flamel and John Dee for a book that will give immortality and unlimited power to whoever owns it. After a fierce battle in a bookstore, the twins flee for safety with Flamel. But is he really concerned for their well-being and the survival of humanity or does he have a much darker agenda, as Dee suggests? Lots of action, lots of great fantasy.