By Madeleine L'Engle.
I don't actually understand why I remember this book as being so important. I associate it with my realizing that I was a committed science fiction reader; but I remember definitely that I'd been reading science fiction routinely for years before I was exposed to this book. (I didn't actually read it, the first time; it was read to us aloud by my sixth-grade teacher.)
It's a fine book; in particular the adolescent characters seemed to me very real.
There are two additional books directly in the series; these aren't bad, but thery're nothing like as good. And L'Engle has written a lot of books that aren't SF or fantasy. I haven't read these, but Pamela likes many of them. Apparently L'Engle is quite strongly Christian in some of those books.