When I was a kid I read A Wrinkle in Time by Madeleine L’Engle. I always wanted to read the other books once I was finished with that one, but my mom told me the other books were really weird.
Finally, a couple of weeks ago I read A Swiftly Turning Planet, which I think is the second one.
In this book, Charles Wallace travels through time with a unicorn, influencing history so that the world does not get blown up in the immediate future. His sister travels along with him by telepathy, but I really think the only reason that she does that is to remain a main character. There seems to be little point to it actually.
This book was just as weird as my mother warned. However, it was also interesting. Madeleine L’Engle seems fascinated by the fight between good and evil that people are largely unaware of. I would not really call this a religious book, but her views on religion and the spirit world definitely come through.
For this reason I recommend this book for children over 12 or 13, and for any adults who like to think about what may or may not be out there.