This is a series of children's books, set in modern day America. The first series (five books) is narrated by Percy himself. You follow him on his various quests as he faces off against some very familiar Greek characters: the Titals (including Atlas), the Furies, the Fates, the Minotaur, and so forth. It's loads of fun and Percy is a humorous narrator. Of course, it being based on Greek mythology, there's going to be lots of tragedy and pain.

Heroes of Olympus takes place a few months later. This time Roman mythology is added to the mix. The story is told in third person with dividing viewpoints between three characters. Until you get to Mark of Athena.

Basically if you like ancient mythology mixed with the modern day, you'll love this book series! And don't watch the movie, it's horribly misrepresenting.