The Time Paradox (Artemis Fowl, #6)

When Artemis Fowl's mother contracts a life-threatening illness, his world is turned upside down. The only hope for a cure lies in the brain fluid of the silky sifaka lemur. Unfortunately, the animal is extinct due to a heartless bargain Artemis himself made as a younger boy. Though the odds are stacked against him, Artemis is not willing to give up. With the help of his fairy friends, the young genius travels back in time to save the lemur and bring it back to the present.