The Zac Power Test Drive series: these are great little books, very easy to read, but full of action. Zac Power is a 12 year old boy whose family members are all spies. Zac's job in this series is to test drive new spy equipment for the spy organisation GIB. http://www.zacpower.com . The website has games and other cool stuff.
The Zac Power Mega Mission series: same characters but for older kids. Zac goes on missions in this series, in one he rescues his long lost grandpa from the clutches of an evil organisation, in another he has to defeat an enemy but he doesn't know who the enemy is, and so on.