A Wrinkle in Time by Madeleine L'Engle. I solemnly told her I wasn't interested in science fiction because it was all about "spacemen and blasters". Where I got that I can't imagine. I had been watching every science fiction film I could find on late night and Saturday morning television for years. Maybe I just didn't want to admit I was a big fan of something so unusual for a girl.
A Wrinkle in Time was written in 1962 and I read it just a few years later. Meg was a wonderful character for me to meet when I did. I have often gone back and read her adventures over the years and the story of her bravery and love are just as powerful with each re-reading.
There is a site celebrating Madeleine and her many works, visit it here