The Lightning Thief
by Rick Riordan
Ares is the God of War, and he is Zeus's son. We first meet him in Denver when he rolls up in a Harley outside of the diner where Percy, Grover, and Annabeth are about to eat. Ares gives them a mini-quest. He wants them to retrieve his shield which he left at a water park in Denver. Ares is having an affair with Aphrodite, and Aphrodite's husband, Hephaestus, loves to publicly embarrass them being all lovey-dovey. Ares knows that Hephaestus has rigged the water park to trap the lovers.