Personally, I really enjoyed reading Tomorrow, When the War Began. For those of you who did to, here is my own list of other books or series that you may enjoy:
I Am Number Four series by Pittacus Lore. A group of teenagers from a different race called Lorian, are fighting their enemy, the Mogadorians on planet Earth. Full of action and friendship, this is an exciting book!
The Gone series by Michael Grant. This is book is similar in that one day, everyone over the age of fifteen disappears, leaving the kids to fend for themself. With a mysterious plot, this series is an interesting read.
Recon Team Angel: Assault by Brian Falkner. The world is at war with an alien race called the Bzadians. It's up to a group of teenagers from around the world to go undercover and find their way into a secret Bzadian facility in Uluru....
Alpha Force series by Chris Ryan. This series is about five teenagers who form an elite young SAS team. Each book is about a different mission, but all are full of action and danger. This group is a force to be reckoned with!
Bodyguard: Hostage by Chris Bradford. Connor Reeves is fourteen and trained in unarmed combat, surveillance, hostage survival, protection techniques. He's the perfect material for a bodyguard. When asked to protect the President's daughter, all his skills are put to an ultimate test...