By Rick Riordan
The real Greek myths, retold by Percy Jackson himself in his wisecracking voice, so a child gets the source material the whole series is built on, with the jokes intact.
Here Percy sits your child down and tells them the actual Greek myths, from the messy creation of the world through the story of every major Olympian. It's not a novel; it's the mythology itself, chapter by chapter, god by god, in Percy's sarcastic modern voice, with John Rocco's gorgeous full-color paintings throughout. This is where a child learns who all these gods really are, Zeus and his temper, Hera's grudges, Hades and the Underworld, before or alongside the novels that use them.
For a child building toward Percy Jackson, this is the grounding: it hands them the mythology the novels constantly reference, so later jokes and monsters land instead of flying past. Percy's voice makes myths that could feel like homework funny, and the book is dip-in-anywhere, one god at a time, so it doesn't demand a straight read-through. A child who loves it will walk into the novels already fluent in who's who on Olympus. Because these are the true myths, though, it's franker than the novels, which is why it sits mid-trail rather than at the gentle end.