Madeline L'Engle

A Wrinkle In Time
A Wind in the Door

These two books, for me and many others, were the books that brought me into science fiction wholeheartedly. These two wonderful books, which to be honest, I can only vaguely remember now, it has been so long since I last read them, were without question the most formative influence on my reading habits today.
If you have not read these books, they are worth going back and reading, even if you are older. If you know kids who like to read, and you want to make them love science fiction, give them a copy of A Wrinkle in Time (I actually prefer A Wind in the Door, myself, but everyone should start with the original...).
These books are truly classic Science Fantasy, and are without question one of the fondest memories of my childhood.
Damn, now I'm going to have to go buy new copies and read them again.