There's not many books that "stunned" me as such. Off the top of my head:
The first two Gormenghast novels immersed me the way few books have since. I probably read them at just the right age (16), but the atmosphere and characterisation, along with Peake's style, were fascinating. I found Dune deeply engrossing for similar reasons.
Neuromancer shocked me, perhaps for the wrong reasons. I remember getting a very bland version of it with a plain yellow cover out of the library when I was 12 or so, and SF involved nice people flying to new planets and perhaps zapping a few monsters. It was the first time I'd seen swearing, sex and drug use in an SF novel. That and the bewildering descriptions of cyberspace made it really puzzling.
I wouldn't say "stunned", but 1984 and Orwell's other works were a real eye-opener, because for the first time I read literature that seemed to be for people like me. Up till then, most literature had seemed to involve the distant past, wealthy people being sensitive in Oxbridge, or the absolutely destitute: here was a guy going out of his way to sing the praises of normal life in my own country. His view was not just that being a normal person in suburban England was not just acceptable but praiseworthy, but that good prose came from honesty, not artifice. Sometimes, even now, those still feel like quite subversive things to say.