The Sea of Monsters
Percy Jackson has made it to the last day of school without getting expelled. Made a friend of a homeless kid, Tyson, too. Except that he's dreaming that Grover is in trouble, and on the last day, monsters come to the school. . .
With Annabeth's help, he escapes, and he and Tyson end up at Camp Half-Blood, at which point it is revealed that Tyson is a cyclopes. And Percy's half-brother. Meanwhile, the guardian pine is dying, and Chiron got the blame, so the new activities is Tantalus. (Did you remember that his punishment was for killing his own children and offering them to the gods as a meal?)
Percy and Annabeth manage to stage the revelation that they know where the Golden Fleece is so that Mr. D and Tantalus are forced to have a quest -- but they give it to Clarisse. And after she visits the Oracle, she sets out alone.
The rest of the story includes Hermes's giving Percy some advice and a bottle of multivitamins, the ship Queen Anne's Revenge, the Sirens, hiding under sheep, guinea pigs, finding a donut store in the forest, a Confederate Civil War ship, Clarisse's prophecy, and more.
The Titan's Curse
Opens Percy's mom driving him and friends to a school -- to rescue students from monsters. Two students, in fact, siblings Nico and Bianca, and they are threatened by a manticore. Annabeth is captured, and the rest are rescued by someone new -- Artemis leading her Hunters, young women -- mortal, demigod, nymph -- who pledged themselves to her. Leading to their coming to camp and the observation that there were some interesting times last visit. (Apollo brought them. While I can generally accept the gods playing down their divine state, given their godly forms can burn mortals to a crisp, Apollo he pushed a little too far into silliness.)
There, Artemis leaves them. Somewhat later, the Oracle reveals she's been captured, and that Hunters and campers have to work together to save her. Percy is not chosen, but one of the chosen ones is put out by a trick from the Hermes kids, and Percy tracks them, and warns them they are being chased by a van when they reached Washington DC.
The tale involves a sea cow, a junkyard where you really don't to pick up anything, the secret past of the Hunter Zoe, the Hoover Dam, advice from Aphrodite, trouble with skeletons, and more.
I do, however, wish to observe that a character who uses thees, thous, and thys should have no trouble with "you." "Thou" is the second person singular; if you know it, you know the plural, too. It's like having someone who knows how to use "I" but not "we."
The Battle of the Labyrinth
Opens with Percy attending an orientation at his new school. I dare say you will not be surprised when monsters attack. . . . oddly enough, one mortal girl seems to see them.
He gets to camp, nonetheless, and finds Chiron has a new instructor: Quintus, a demigod who actually managed to grow to adulthood. They learn that Luke is trying to master the Labyrinth. Like all of Greek mythology, it migrated, and is now under the United States. And one end leads to the camp, past all their defenses. All he needs is Ariadne's thread.
Quest time. It involves Annabeth's getting a wish and being told she already has what she wished for, and Percy knows it; a ranch; a son of Ares actually saying that he won't fight; a volcanic explosion; Clarisse's softer side; Nico's buying Happy Meals for the dead; gladiator games; and more.
The Last Olympian
A drive on the shore is interrupted by a pegasus arriving with fellow demigod Charles Beckendorf. They set out to blow up the Princess Andromeda, filled with Kronus's forces. Well, they do -- and Percy makes it back. Much to the grief of Silena, Beckendorf's girlfriend. (The gods not exactly having human DNA, the kids really only mind having the same godly parent, romantically.)
The war is heating up in other ways, too. Poseidon is deep in battle undersea. Typhon breaks free of Mount St. Helens, and all the gods are fighting him. Leaving the campers to do stuff. . . it would be a little better if Clarisse wasn't insisting the Ares cottage had been insulted and would not fight.
War ensues. It involves some automatons of Daedulus, the goddess Hestia keeping the hearth burning, some campers being easily taken in by something they wanted to believe, using a sanddollar to win over river gods, and more.