A Wind in the Door is even weirder than A Wrinkle in Time. I mean, we can dig space travel and other planets, sure, that's sci-fi, even if how it's framed is rather unconventional. But A Wind in the Door is an even more fantastic Fantastic Voyage. Young Charles Wallace, beloved younger brother of Meg Murry, is sick, a strange disease afflicting him on a cellular level. But that doesn't concern him as much as the dragons he saw in the vegetable garden. When he, Meg, and their friend Calvin go to investigate, they find that the dragons are actually a cherubim, and with his help and the help of a Teacher, they must travel inside Charles Wallace's mitochondria and use a form of telepathy to defeat the Echthroi, the force of nothingness who want to wipe out all goodness and existence in the universe.
For all that, it's an enjoyable follow-up to A Wrinkle in Time, returning to the same beloved characters, especially stubborn and dramatic Meg. There are some weird concepts, but the writing eases you into them, and you never feel like L'Engle is condescending to you. The book makes sense, but it's really something you experience, just letting yourself be wrapped along in the joy of the triumph of good and love.