I, too, think that Michael Scott Rohan is an under-rated writer -- and I agree about his descriptions of the Ice. I've been trying to get two of the books in the second trilogy forever, but when I find a copy of either one on any of the amazons its always at a price I can't possibly afford.
But as for books that stunned me -- when I was very young, so many books did that (I was a very receptive reader, easily moved) none of them stand out. I, too, love The Count of Monte Cristo, but I first read it (probably abridged) when I was ten and, as I said, so many books affected me in those days, I can't remember if that one had an exceptional impact.
As an adult -- or nearly an adult when I first read it -- I would have to say The Lord of the Rings. Back then, in 1966 or 1967, I had no conception that there could be anything like it.
A Christmas Carol by Charles Dickens. I thought I knew it, but when I read it for the first time as an adult I was amazed by the way some of the scenes touched me on such a deep level.
Lud in the Mist by Hope Mirrlees, because it immersed me so completely in its little world. I had thought fantasy needed epic scope to do that.
A lot of books hit me smack between the eyes on a second or even a third reading, because I find they are so much better than I thought they were the first time. That list would be a very long one.