Percy Jackson and the Olympians by Rick Riordan is my current favorite children's book series. I'm a children's librarian, so that is saying something. The Lightning Thief is the first book.
This series is Harry Potter meets the Greek Gods and Goddesses. The Lightning Thief explores twelve-year-old Percy's first adventure after he discovers there is something special about him; he is the son of one of the Olympians. The Olympians have so many children with humans that they developed a special summer camp for them called Camp Half-Blood. Each God or Goddess has their own dorm where their children stay. The rivalries that spring up between the actual Olympains also happen to their children, which makes for great competition between the groups.
Of course there is a larger, specific quest that Percy must deal with in this book, but you'll have to pick it up to read more.
This book is very exciting and fast-paced. Percy's friends are loyal and funny. My favorite is Grover who is quite possibly the perfect best-friend side-kick I've ever seen.
There are currently 2 other books in the series with many more to come.