I think it's a mix of simplicity and a specific kind of energy from the mid-2000s. The covers aren't overly detailed or busy; they're focused. You get this bold, central image—the lightning bolt, the trident, the ship's mast—against a solid, saturated color background. It feels like a symbol, not a scene. And the font for 'Percy Jackson' is so distinct, that rough-edged, slightly weathered type. It doesn't look like a fantasy epic cover; it looks like the logo for something cool and slightly dangerous a kid would find. That visual shorthand told you exactly what you were getting into before you even read the blurb: modern, mythic, and paced like an action movie.
What seals it for me is the contrast. The ancient symbol rendered in this clean, almost graphic design way. It bridged the gap between the old myths and a contemporary middle-school story perfectly. Later editions with photo-realistic covers or movie tie-ins never captured that same iconic, almost trademark-like feel. The originals were instantly recognizable on a library shelf or a classroom floor. They promised a specific adventure, and they delivered.