The Battle of the Labyrinth by Rick Riordan

In The Battle of the Labyrinth, Percy finds out that Kronos is planning an attack that  he's leading himself through the Labyrinth! Now, to save Camp Half -Blood, Percy and some of his friends must go through the Labyrinth, face tough obstacles, and find Ariadne's string before Kronos does before it's too late!

The Titan's Curse

In The Titan's Curse, Percy, Thalia, a girl who came back to life after the Golden Fleece was put on her as a tree, and Annabeth find Grover to help bring back two demigods in a school with a monster. Meanwhile, they encounter Artemis and her noble hunters when Annabeth gets kidnapped! Now, Percy and the Hunters must rescue Annabeth from San Francisco to save his friend!