If you liked The Lightning Thief, check out the sequel: The Sea of Monsters. There are 5 books in this particular series, but Rick Riordan has over 30 books that you'll probably love!
When Thalia’s tree is mysteriously poisoned, the magical borders of Camp Half-Blood begin to fail. Now Percy and his friends have just days to find the only magic item powerful to save the camp before it is overrun by monsters. The catch: they must sail into the Sea of Monsters to find it. Along the way, Percy must stage a daring rescue operation to save his old friend Grover, and he learns a terrible secret about his own family, which makes him question whether being the son of Poseidon is an honor or a curse.