A Wind in the Door is a young adult

Young adult literature

Young-adult fiction or young adult literature , also juvenile fiction, is fiction written for, published for, or marketed to adolescents and young adults, roughly ages 14 to 21. The Young Adult Library Services of the American Library Association defines a young adult as "someone between the...

 science fantasy

Science fantasy

Science fantasy is a mixed genre within speculative fiction drawing elements from both science fiction and fantasy. Although in some terms of its portrayal in recent media products it can be defined as instead of being a mixed genre of science fiction and fantasy it is instead a mixing of the...

 novel

Novel

A novel is a book of long narrative in literary prose. The genre has historical roots both in the fields of the medieval and early modern romance and in the tradition of the novella. The latter supplied the present generic term in the late 18th century....

 by Madeleine L'Engle

Madeleine L'Engle

Madeleine L'Engle was an American writer best known for her young-adult fiction, particularly the Newbery Medal-winning A Wrinkle in Time and its sequels A Wind in the Door, A Swiftly Tilting Planet, Many Waters, and An Acceptable Time...

. It is a companion book to A Wrinkle in Time

A Wrinkle in Time

A Wrinkle in Time is a science fantasy novel by Madeleine L'Engle, first published in 1962. The story revolves around a young girl whose father, a government scientist, has gone missing after working on a mysterious project called a tesseract. The book won a Newbery Medal, Sequoyah Book Award, and...

, and part of the Time Quartet

Time Quartet

The Time Quartet/Quintet is a fantasy/science fiction series of five young adult novels written by Madeleine L'Engle.Those novels are:*A Wrinkle in Time , , ISBN 0-374-38613-7*A Wind in the Door , ISBN 0-374-38443-6...

 (and by extension the Time Quintet).
Meg Murray is worried about her brother Charles Wallace, a 6-year-old genius who is bullied at school by the other children. The new principal of the elementary school is the former high school principal, Mr. Jenkins, who often disciplined Meg, and who Meg is sure has a grudge against her whole family.