Rick Riordan's Percy Jackson and the Titan's Curse is the third book of the five books in the Percy Jackson series.
As you will know, if you've been following this series, the Big Three Olympian Gods (Poseidon, Zeus and Hades) all agreed after World War II to stop siring half-blood children with mortals as such Half-Bloods generally cause major problems for the world (such as WW2!). Unfortunately 14 year old Percy Jackson's dad reneged on the deal and Percy's the Half-Blood son of Poseidon). He's not the only one who was guilty of breaking the agreement though. Zeus also broke it and his daughter, Thalia, never made it safely to Camp Half-Blood, where the children of gods and mortals are trained to cope with the monsters that beset all Half-Bloods. As Thalia was dying, her father turned her into a pine tree and she stood guard over the Camp for many years until Luke, son of Hermes, poisoned the pine tree. Percy and his friend Grover the Satyr managed to recapture the Golden Fleece and bring it back to heal the tree which magically protected the camp, but not only did the Fleece expel the poison from the pine tree, it also expelled Thalia. This happened a few months before The Titan's Curse opens (at the end of The Sea of Monsters review).
Thalia is still getting used to being amongst people again, but she and Percy have become friends, and the two, together with Percy's friend Annabeth (daughter of Athena), set out on a rescue mission to help Grover get two newly-discovered Half-Bloods from the military boarding school where they live to Camp Half-Blood. Unfortunately, one of the school teachers is really a Manticore and he objects to Nico and Bianca di Angelo being taken from the school. During the battle that ensues, Annabeth is captured, despite the timely arrival of Artemis, Goddess of the Hunt and her Hunters. Artemis offers Biana the chance to become a Hunter and she agrees. Afterwards Artemis sets off alone to hunt another monster that is destined to cause the downfall of Olympus, whilst Percy, Grover and Thalia, together with the two Half-Blood siblings and Artemis' Hunters, travel with Apollo the Sun God to Camp Half-Blood for their safety.
Back at Camp, The Oracle delivers a prophecy that leads to a team of five Campers being chosen to go in search of Artemis, who has gone missing. Thus Percy, Thalia, Bianca, Zoe (another Hunter) and Grover set out on a journey that takes them across the country from East Coast to the West, to San Francisco where Percy finds out, amongst other things, what it's like to carry the weight of the sky on his shoulders. Cool monsters, skeletal warriors and bad guys abound in this tale, as well as the Gods behaving badly (and occasionally making bad haiku - Apollo). Percy gets to meet Aphrodite, they visit the Hoover Dam, two of the party of five are killed, then finally, the Gods vote on how to kill the three survivers of the quest!
Percy Jackson and the Titan's Curse is also available from Amazon.com - I must say, I do like this blue cover on the US edition !