_A Wrinkle in Time_ by Madeleine L'Engle
I really enjoyed it, and can see why it's a classic. There are some religious undertones that grated on me a little, but in fairness, I get unreasonably prickly about that sort of thing, and in my more objective moments I can see that they're minor and don't get in the way of the narrative flow or themes of the story.
At its heart, it's a story about fierce, uncompromising love, and understanding that "different" doesn't mean "wrong." Yes, there's also travel through folded space, alien worlds and creatures, and an eternal war against a darkness that seeks the subjugation of all sentient races. And, while that's fascinating and compelling, the story is really about a misfit girl named Meg, and the way she'll move heaven and earth to save her father and brother, and make her family whole again.