The Lightning Thief by Rick Riordan
Chapter 6: I Become Supreme Lord of the Bathroom

This is the first real glimpse of what would be the staple setting of the Percy Jackson series–there isn’t a single book that doesn’t have Camp Half-Blood in it, even if they’re just passing through. Because it’s just a glimpse, most of what Percy sees and learns is on a surface level with an obvious under-layer hidden away.

The Lightning Thief by Rick Riordan
Chapter 5: I Play Pinochle with a Horse

This is the necessary exposition chapter. Here, Percy learns about Greek mythology being real and is introduced to a couple of new characters and reintroduced to one old character.