The Lightning Thief  by Rick Riordan....

  When his lightning bolt is stolen, Zeus accuses Poseidon's son Percy Jackson and gives Poseidon's son fourteen days to return it, otherwise he will initiate a war amongst the gods. Meanwhile the teenager, Percy, who is dyslexic and has ADHD is visiting The Museum of Metropolitan of Art and is attacked by a Fury disguised in his teacher. His crippled best friend Grover reveals that Percy is a demigod and that he is his protector and his teacher Mr Brunner gives him a pen telling him that it is a powerful weapon. They go to Percy's house and together with his mother Sally they drive to the Camp Half-Blood. However Sally is attacked by a Minotaur and vanishes before Percy can help her. In the camp, Percy befriends the gorgeous Annabeth; when they are attacked by Hades who wants the lighting bolt for himself, Percy discovers that his mother is in the Underworld with Hades. Percy decides to travel on a dangerous quest to retrieve the lightning bolt and save his mother.

The Sea of Monsters  by Rick Riordan....

In The Sea of Monsters, a 13-year-old demigod Percy Jackson goes on a quest with his friend Annabeth, daughter of Athena. They go to save Thalia's pine tree, which has been poisoned mysteriously , and to save Grover, Percy's best friend. In order to get the Golden Fleece, which will heal the tree, he must travel across the omious Sea of Monsters to an island guarded by the Polyphemus and his man-eating sheep, where his friend Grover is trapped. Throughout the journey, Percy, Annabeth and Tyson face many challenges, such as the Sirens, Scylla, and Hydra.  In the end, Percy rescues Grover, defeats Polyphemus (with help from Tyson, who was thought to be dead by that time), and retrieves the Golden Fleece. Chiron is re-hired and everything seems to be perfect. When they return to camp, the camp places the Fleece on the tree. The next day, Grover awakens Percy to find out that the Golden Fleece also brought Thalia back to life, giving Kronos another chance to control the prophecy.