I gather this book is very big in Australia, but sadly I rarely see it mentioned in N. America. Here's hoping the recent Scholastic release will help correct that! :) I feel lucky to have stumbled across it several years ago:
What it's all about:
When Ellie and her friends take off for a week of camping and goofing off in a remote valley near their town, they hardly expect their entire world to change in the meantime. But on returning, they find the outlying farms abandoned and pets and livestock dying. An enemy army has invaded their town, imprisoning their families and patrolling the now-unfamiliar streets. There doesn't seem to be much a group of teens could do. But faced with the alternatives of giving up or running away, Ellie and her companions choose to fight.
Why it is awesome:
Engaging characters, intense action scenes, and a compelling voice that sucks you right in--what else could you ask for? Well, this book gives even more than that. Despite how "big" the events are (entire town--and probably the entire country--taken over in a mass invasion), the story itself always feels as though it could really be happening. Ellie and her friends may fight back, but their skills and strategies are those actual teens would have--they don't magically turn into super-soldiers. Every loss is deeply felt, and every gain comes at a price. Exactly what I want in this sort of story: both exciting adventure and brutal realism.