Artemis Fowl and the Time Paradox is the sixth book in the criminally good Artemis Fowl series by Eoin Colfer.

FORMER CRIMINAL MASTERMIND ARTEMIS FOWL HAS HOODWINKED FAIRIES AND BATTLED GOBLINS
THIS TIME, IT'S PERSONAL
Artemis's mother is gravely ill. Unfortunately the cure, an endangered lemur, was made extinct eight years ago . . . by Artemis Fowl. Talk about awkward.
What if he could travel back in time, outwit his former self and bring the lemur home? With the right demon warlock, it might be possible.
But we all remember the young Artemis. He was, to put it mildly, a pretty terrible human being, so this won't be easy.
Artemis Fowl really is his own worst enemy . . .