The Lightning Thief by Rick Riordan
This Lightning Thief is a lot of fun, full of exciting action and humorous characterizations of Greek gods and goddesses. The premise is that those gods really exist, that Mount Olympus now resides on the 600th floor of the Empire State Building (invisible to mortals), and that the gods are still carrying on just like they did in the old days. In fact, they've got some half-blood offspring running around in the world, kids of a god or goddess and a mortal. It won't teach you much beyond the most basic Greek mythology, but it'll remind you of some great stories. My boys loved this book and the sequels (more about them later).