Well I am forever grateful to David Gemmell. Read Dark Moon standalone when I was 12 and this was the beginning of my love affair with the Fantasy genre - after I read this I started reading everything Gemmell had written up to that point (All the Drenia, Rigante and standalones.) This was the transition for me between reading Goosebumps, Narnia, Dark is Rising etc. to more adult novels.
I pretty much read Fantasy for the next 4 years or so - Gemmell, Goodkind (Please don't get me started on this pile of turd), Feist, Tolkien, Donaldson etc.. When I was 16 I read Enders Game and this was the start of my love for Sci Fi. Enders game was a fantastic book.
More recently, as an adult, TSA (for me TWP was the highlight) and Hyperion have both stood out in terms of quality and showed me how well planned and written a book can be. Hyperion is just so well written and so easy to read - the story seems to tell itself so well, TSA on the other hand can be a difficult read but rewards the reader immensely with some of my favourite prose and style from Bakker.
So nothing that has changed my life - more a change in reading habits. :)
Edit: In retrospect I think I should add Lord Fouls Bane as well:
Thomas Covenant raping Lena was just an awful scene and I read it when I was fairly young and it was the first time I had read this tone from the protagonists PoV. Changed the way I read characters quite a bit