Percy Jackson and the Titan's Curse by Rick Riordan

Percy, Annabeth, Thalia and Grover was on a mission to bring two demigods, Bianca and Nico, back to camp when a little incident happened. They nearly got killed by a manticore when the Hunters, lead by Artemis - sister of Apollo, rescued them. Unfortunately when the manticore jumped down the cliff, he took Annabeth with him.
The manticore mentioned something about the Stirring Monster and Artemis decided to go find and kill the monster. She left the Hunters and told them to go to half-blood camp. They were back to camp for several days when Percy dreamt about Annabeth was tricked to hold a ceiling from falling down by Luke who betrayed the camp and follow the Titans and Zoe Nightshade, Artemis' lieutenant, dreamt about Artemis in trouble. Later Percy found out that the Titans used Annabeth as a bait to trick Artemis to hold the ceiling, and the ceiling is atually the sky which used to be hold by Atlas.

Percy, Thalia, Grover, Zoe Nightshade and Bianca, who joined the Hunters, went on a quest to safe Artemis and Annabeth. They got a prophecy that 5 would leave the camp to go on the quest but only 3 would return.