The Titans Curse
By Rick Riordan
This book is the last in it's series. It is my favorite in the series. It includes a long grudging battle between the forces of the titans and demigods. Percy meets many foes, including some he has already defeated. His enemy Luke is leading the charge. What he doesn't know is that, Luke has been possessed by the titan lord Kronos. Percy defeats many enemies with the help of his friends. They even manage to turn one into a tree.