Lord of the Rings. Read it in 3 days (afternoons after school and evenings to late nights), felt... alive/awaken, during and after reading it. I was maybe 12 years old.

1984. First real sense of society at large, totalitarianism, and other "grown-up" stuff. I was maybe 12 or 13.

Brave New World. Same as 1984, except I was maybe 14 or 15.

1491 and 1493. Amazing way to deeply understand history, and completely new perspectives on America pre and post Columbus.

The Pillars of the Earth. First time a novel captured me so deeply, in terms of sense of history, and sense of other people's lives.

Then, various novels, mostly from Italian authors (I grew up in Italy).