Percy Jackson - The Battle of the Labyrinth

In book four, a door to the Labyrinth of Daedalus is discovered in the woods of Camp Half-Blood. The camp is in danger of being invaded by Kronos' army by means of the Labyrinth. Chiron sends Annabeth along with Percy, Grover, and Tyson into the Labyrinth on a quest to find Daedalus, in hopes that they might use his knowledge to help thwart any attacks that are carried out using the giant underground maze. They eventually find Daedalus, who has made himself a new body, and has disguised himself as the swordplay counselor.
Hera opens a door to Mount Othrys (Mount Tamalpais), where the Titans are staying until Kronos gains a new body and is able to invade the camp. Annabeth gives Percy her magic invisibility hat and he goes up to investigate. There, he sees a coffin. He flips the top off of the coffin and is stunned to find that it contains the body of his old enemy, Luke Castellan. There is one small hole in his body right where his heart was, because he is consuming amounts of power. Ethan Nakamura, a son of Nemesis, is the last demigod to join the Titan lord's cause. When he pledges to renounce the gods and join the Titans' army, the last piece of Kronos is risen from Tartarus. Luke's body rises out of the coffin, the hole healed, and he is stronger than ever.
There is a battle, as Kronos' army attacks Camp Half-Blood through the Labyrinth. Many are injured and some are left dead. Daedalus shows up to help along with his giant pet hellhound, Mrs. O'Leary. Daedalus has escaped death for centuries, but gives himself over to Nico di Angelo, a son of Hades, so he can accept his punishment in the Underworld, and also so he can see his son Icarus, who was accidentally killed by one of his own inventions, and his nephew Perdix, whom he had regrettably killed out of rage. Percy inherits Mrs. O'Leary, who seems to prefer him to any other demigod. Percy and Annabeth's relationship starts to change. Annabeth saves Percy several times and kisses him once because she thinks he is going to die. She also makes a big scene in front of all the campers when she realizes that Percy is alive.
This is a romantic book for Percy and Annabeth as they hug and kiss. Percy still hasn't told Annabeth that Aphrodite has said they are in "such a cute relationship it makes her want to cry.