The Time Paradox (Artemis Fowl, Book 6) USED YA

Artemis's mother has contracted a deadly disease--and the only cure lies in the brain fluid of African lemurs. Unfortunately, Artemis himself was responsible for making the lemurs extinct five years ago. Now he must enlist the aid of his fairy friends to travel back in time and save the lemur. Not only that, but he must face his deadliest foe yet: his younger self.