A Wind in the Door by Madeleine L'Engle

I finished this book a few weeks ago. I read it back in high school, and I thought I would give it another read. I enjoyed it very much, but its predecessor "A Wrinkle in Time" was better. I like the themes in the book, such as working for the greater good and putting others first. The story is largely about Charles Wallace, a young boy who is seriously ill because his mitochondria is not working properly. There exists in his cells a device called "farondolae" that help mitochondria work.