Tomorrow, When the War Began
When I was twelve I picked up a book, not because I had any interest in reading it but because I was forced to read something (good old SSR). The book was Tomorrow, When the War Began and it instantly became the favourite book of my youth. I hit the library the same week a took out every book in the series that had been released so far. Since then, I have read every book in the Tomorrow series and it's spin off The Ellie Chronicles.