I read the His Dark Materials trilogy in middle school, and that hugely influenced my thinking about power structures generally, religion specifically, and morality broadly. I mean, I didn't have words for a lot of that stuff at the time, but looking back they were massively influential.
I re-read them a couple years ago and they really hold up astonishingly well. And yes I am secretly smug about having read these books as a kid while most of my peers were reading Harry Potter.