Your child is hooked on Greek gods and monsters and you want to feed it without dropping a fat, frightening tome on them. Start at the top and read down — this is a ladder, and each book asks a little more of your child than the one before. You begin with the gods as short, silly chapter-book fun, move through a jokey podcast-style romp and Percy Jackson's own take, and finish with two beautiful, grown-up myth collections a confident reader can sink into. By the last rung they'll know the myths cold, and they'll have grown into a real classic to get there.
What climbs across this trail is reading difficulty, and it climbs on purpose — this is a ladder, not a shelf, so read it top to bottom. Heroes in Training hands a child the Greek gods as short, cartoon-light chapter-book fun. Goddess Girls steps the reading up a notch while keeping the myth cozy, the Olympians as middle-schoolers. Greeking Out brings in the real myths at last, but told at podcast speed with a joke every other line, so the violence arrives cushioned by comedy. Percy Jackson's Greek Gods gives them Riordan's own funny, fuller telling — the grounding the novels assume. And then the two collections: Napoli's Treasury tells the myths as literature, with weight and gorgeous paintings, and the d'Aulaires' classic is the fullest and most demanding of all, a real 1962 book a child grows into. Each rung is a little harder than the last, and by the bottom your child hasn't just been fed their obsession — they've been pulled up a whole reading level by it.