The Green Door is a novel written by Mildred A. Wirt, first published in 1934. The story follows the adventures of the young and adventurous Ruth Fielding, who moves to a small town to attend college. She rents a room in an old mansion with a mysterious green door, and soon discovers a book hidden behind it. As she delves into the book's secrets, Ruth finds herself embroiled in a dangerous mystery that puts her life in peril.

Mildred A. Wirt, who wrote the book under the pseudonym of Joan Lowell, was a prolific author known for her mystery and adventure novels for young adults. She wrote over 130 books, including the popular Nancy Drew series. The Green Door, while not as well-known as the Nancy Drew books, was a hit in its time and has remained a beloved classic among fans of young adult literature.

The characters in the book are well-drawn and engaging, particularly Ruth Fielding, who is a strong and resourceful protagonist. The book's plot is full of twists and turns, and Wirt's writing is lively and engaging, drawing the reader into the story from the very first page. The Green Door is a testament to Wirt's skill as a writer and her ability to create compelling stories that capture the imagination of readers of all ages.