If you know anything about Greek mythology, you'll know that their gods had a tendency to produce demigod kids by the dozen. And the "Percy Jackson and the Olympians" series is all about a boy who discovers that he's the offspring of a god, and the other demi-god kids that he gets to know at a very, very special camp. Rick Riordan spins a clever fast-moving adventure that mines ancient mythology and gives it a modern spin.
Percy Jackson has always been a troublemaker, but he's shocked when some truly strange things begin to happen in his life. After a minotaur attack, he learns that he's at a special training camp called Camp Half-Blood intended for demi-god children, and that his best friend Grover is actually a satyr bodyguard. Though Percy is understandably resistant to the idea, he soon makes friends in the sharp Annabeth and the bitter Luke (and enemies with the kids from Cabin Ares).
Oh yes, and he finds that he's the son of the god Poseidon... which is a problem since the "Big Three" gods have sworn an oath not to father any more kids, due to a rather ominous prophecy.
As a result, Percy finds himself in the middle of some epic cosmic battles -- he must recover Zeus' master lightning bolt before the world is thrown into chaos; he must venture into the Sea of Monsters to get Grover and the Golden Fleece; clash with the Titan Atlas; embark on a journey into the legendary Cretan Labyrinth to save the camp; and finally battles against Lord Kronos to save Olympus.
The Percy Jackson series is all a little Harry Potter in concept -- ordinary kid discovers he has magical powers, and is taught in a specialized school/camp while fighting against the forces of evil. Rick Riordan spends the first half of "The Lightning Thief" exploring the whole idea of Camp Half-Blood and the demi-god kids -- but after that, the series pretty much flows quickly to the slam-bang end.
Riordan has a snappy fast-moving style, and he peppers the story with plenty of plot twists and monstrous action. And he has quite a sharp-edged sense of humor -- the snarkiness is a bit annoying in the first chapter, but after that he produces some fun dialogue ("With great power... comes great need to take a nap"). And he does a good job with the concept of gods and monster surviving over the center of the western world, as well as spoking some fun at the gods' behavior.
I found Percy rather annoying in the first couple chapters, but Riordan slowly evolves him from a rather bratty, rebellious kid to a reluctant budding hero who will take on literally anything to keep his loved ones safe. Annabeth is an excellent counterpart to Percy, smart and measured if rather haughty in attitude, as well as the timid Grover, embittered Luke, punky Thalia and moody Nico. And the gods are portrayed in a rather clever, tongue-in-cheek way.
It has a couple slow spots, but Rick Riordan's Percy Jackson and the Olympians series is a solid, fast-moving little fantasy series, with likable characters and plenty of wit.