The New Trinity -- Percy Jackson Batman Crossover -- The Olympian Heroes Book 4

Perseus Jackson and Annabeth Chase are trying to get back into the swing of School, and Social life but it's hard when you're off fighting a new villan every other week. Not to mention the Impending war with the Titans.
Percy and Annabeth find heroes are literally vanishing off the face of the Earth, but Percy finds the Heroes aren't the only ones in trouble. So is the rest of the world.

Book 4 in the Olympian heroes series