Hyperion and the great balls of fire
(Book)

"Under the Titan Hyperion's rule, the sun is burning even brighter than normal, and scorching everything in northern Greece--including the villagers! The Olympians are forced to play an un-fun game of Dodge the Sunbursts as Hyperion hurls giant fireballs that could fry them up! What's worse, it looks like a familiar foe, Cronus, is hot on Zeus, Poseidon, and Hades's trail. Armed with some clues, they set out to figure out why Hyperion has been making things so hot--and to rescue Hera, who is still missing"--