I would have to say... "The Circle" by Laura Day.
It's nothing new to anyone here, but at the time, I was a 17-year-old high school senior, and I had never encountered anything that made me feel so empowered. It was the first book that really opened my eyes. It was like my secret little "window." It kind of became my bible for a couple years, until I needed something new, and found some books with greater depth.