Ms. l'Engle's previous book, A Wrinkle in Time, is an inspiring and profound work, and this is the nth degree above it. The reader can tell that her writing style has matured. She does not slip into a didactic mode which (slightly) mars the last chapter of Wrinkle. The science fact and science fiction are sufficient to satisfy buffs of the genre, and sufficiently subtle not to get in the way of those to whom they do not appeal. (whew) More important, though, is the theme, which reaches and suffuses through your mind. Reading Ms. l'Engle's books was my first truly mind-expanding experience.
It is a red herring to call this work a children's book. What it truly is is a book for those who want to expand their mind and their view of the universe, including children. I first read this book at age 11 and have continued to read it repeatedly up until 17; I expect to continue to reread it for a long time to come.