A Wrinkle in Time by Madeleine L'Engle was one of my favourite books when I was a boy; and I enjoyed re-reading it a couple years ago. But I had no idea that L'Engle wrote a sequel. In fact, she wrote a total of five books featuring this family, which are collectively known as the "Time Quintet".
A Wind in the Door is the second book in the series. Meg is in high school and her brother Charles Wallace - a child prodigy whose intelligence borders on superhuman capabilities - is bullied at school because he is so different from his small-town classmates.
Meg and Charles's mother is studying farandolae - theoretical components of the mitochondria of human cells that are too small to be seen by any microscope. Charles Wallace is losing his farandolae and Meg shrinks down to go inside Charles's body and battle the evil forces that are attacking him in an effort to destroy everything in the universe. Meg is accompanied by her high school principal and assisted by an alien creature. During their ordeal, the team discovers that the farandolae are sentient creatures and that their destruction is engineered by the evil Echthroi, who are determined to bring chaos to the universe.
As with "Wrinkle", this book features an angelic creature who aids the children and a malevolent force bent on destroying the universe. In this case, the angelic creature is a many-eyed, many-winged monstrosity that resembles a group of dragons and the demonic race is the Echthroi.
While lacking the scope of the series’ first novel, this book does an excellent job of relating cosmic turmoil with personal struggles. The potential destruction of everything is reflected in the battle to save Charles. L'Engle combines the excitement of fantasy and science fiction with the normalcy of family life and the power of love in both contexts.