The Lightning Thief

Basically, the premise is the same between this book and the Harry Potter series: our tween hero and his two sidekicks (one a brainy girl, the other a bumbling boy) go on a very dangerous mission to save the world, a mission that for some reason can only be carried out by these three children. A very wise adult advises them (Chiron/Dumbledore), but does not actually get involved in the mission. Also, these children battle crazy-dangerous monsters/adults, and win every time.

So if you've already read HP this will probably feel like been-there-done-that, and it was done better before.

Haven't read HP? Then as long as you can suspend your disbelief regarding the savviness of 6th graders (which is much harder for me than suspending disbelief regarding Greek gods and monsters), you'll probably enjoy it. Taken on its own merits, this book is a pretty good read. Fast-paced, and I love the Greek mythology angle.