For me it was Madeleine L'Engle's time trilogy, and the other Murray/O'Keefe books. Meg Murray and Calvin O'Keefe were very much with me throughout high school, and shaped so many of my thoughts and thinking then.
And now--there are bits of my world view I can still trace to those books. And part of my still hopes--as I did then--to get whisked off to some other world some windy night, by a unicorn or a Mrs. Whatsit or something else entirely.
So I take back any grumpy thoughts I've had the past few days. No one spoil the series this close to the end, all right? (But please don't drown kittens or blow up planets either, okay?)