Two more I wanted to mention. (Edit: This post turned out to be a lot longer than I expected, sorry.)
The first is sort of a counterpart to my mention of Atlas Shrugged earlier in this thread. Faith of the Fallen, by Terry Goodkind, is the 6th book in a fantasy series whose first book, Wizard's First Rule, I greatly enjoyed in middle school. I had continued reading the series, getting diminishing returns on each one, and when I read Faith of the Fallen, I realized that the series either had become or just always was pure shit; it just hadn't been apparent to me until then.
FotF had exceedingly poor characterization of characters I had come to know and love; it turned the main character into a pseudo-Objectivist, John Galt-speech-blathering asshole who was obviously intended to be flawless and used all manner of shoddy plot contrivances to make him seem to. So its first major influence on me was making me realize that people with their heads up Ayn Rand's ass tend to be fucking obnoxious.
Also, after years and years of reading and enjoying sci-fi/fantasy novels that I enjoyed at the time and which were probably vital to my development as a reader but which would probably make me cringe now, I think FotF marks the point where I realized that I had somehow developed some sort of taste in fiction, and that Mercedes Lackey and Xanth books were, quite irrevocably, just not going to do it for me anymore.
That said, the second book I wanted to mention is Infinite Jest, by David Foster Wallace. A more intimidating book I had never seen: between the blurbs on the cover talking about how postmodern or whatever it was and its length (>1000 pages, of which over 200 are footnotes), I thought I was just buying it because it looked intriguing and I'd never actually be able to finish it.
Fortunately I was very, very wrong, and IJ is now one of my favorite books: I sort of started to understand its groove around 200 pages in and for the rest of it I was hooked. DFW is an incredible writer: he's flashily erudite and loves the big words, but he's also flippant and vernacular. Stylistically, there is no one else like him, and IJ was probably the first book to make me really understand what it means for a writer to have a style.
It also changed my life simply by changing my idea of what a novel could be; I doubt there will ever be another like it, but it's pushed me to try other modern and kind of wacky writers to try to find something that can approach its magnificence.