"The Time Paradox" by Eoin Colfer

6th part of the Artemis Fowl series.
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.

The previous five books of Artemis Fowl series are here http://englishtips.org/index.php?newsid=1150792561