A Wind in the Door

In A Wrinkle in Time Quintet book two, Charles Wallace falls ill, and Meg, Calvin, and their teacher, Mr. Jenkins, must travel inside C.W. to make him well, and save the universe from the evil Echthros.
It is November. When Meg comes home from school, Charles Wallace tells her he saw dragons in the twin's vegetable garden. That night Meg, Calvin and C.W. go to the vegetable garden to meet the Teacher (Blajeny) who explains that what they are seeing isn't a dragon at all, but a cherubim named Proginoskes. It turns out that C.W. is ill and that Blajeny and Proginoskes are there to make him well – by making him well, they will keep the balance of the universe in check and save it from the evil Echthros.
Meg, Calvin and Mr. Jenkins (grade school principal) must travel inside C.W. to have this battle and save Charles' life as well as the balance of the universe.
Books by Madeleine L'Engle

A Wrinkle in Time Quintet
A Wrinkle in Time
A Wind in the Door
A Swiftly Tilting Planet
Many Waters
An Acceptable Time
A Wrinkle in Time: The Graphic Novel by Madeleine L'Engle; adapted & illustrated by Hope Larson
Intergalactic P.S. 3 by Madeleine L'Engle; illustrated by Hope Larson: A standalone story set in the world of A Wrinkle in Time.
The Austin Family Chronicles
Meet the Austins (Volume 1)
The Moon by Night (Volume 2)
The Young Unicorns (Volume 3)
A Ring of Endless Light (Volume 4) A Newbery Honor book!
Troubling a Star (Volume 5)
The Polly O'Keefe books
The Arm of the Starfish
Dragons in the Waters
A House Like a Lotus
And Both Were Young
Camilla
The Joys of Love