Answer by taking a quote from the following article:

Garland was born Frances Ethel Gumm on June 10, 1922, in Grand Rapids, Minnesota. She was the youngest child of Ethel Marion (nee Milne) and Francis Avent "Frank" Gumm. Her parents were vaudevillians who settled in Grand Rapids to run a movie theater that featured vaudeville acts. She was of Irish, English, and Scottish ancestry, named after both of her parents and baptized at a local Episcopal church. "

In 1938, she was cast in her most memorable role, as the young Dorothy Gale in The Wizard of Oz (1939), a film based on the 1900 children's book by L. Frank Baum. In this film, she sang the song with which she would be identified, "Over the Rainbow". Although producers Arthur Freed and Mervyn LeRoy had wanted her from the start, studio chief Mayer first tried to borrow Shirley Temple from 20th Century Fox, but they declined. Deanna Durbin was then asked, but was unavailable, resulting in Garland being cast. Garland was initially outfitted in a blonde wig for the part, but Freed and LeRoy decided against it shortly into filming. Her blue gingham dress was chosen for its blurring effect on her figure, which made her look younger. Shooting commenced on October 13, 1938, and was completed on March 16, 1939, with a final cost of more than US$2 million. With the conclusion of filming, MGM kept Garland busy with promotional tours and the shooting of Babes in Arms, directed by Busby Berkeley. Rooney and she were sent on a cross-country promotional tour, culminating in the August 17 New York City premiere at the Capitol Theater, which included a five-show-a-day appearance schedule for the two stars. Garland was forced into a strict diet during filming; she was given tobacco to suppress her appetite. The Wizard of Oz was a tremendous critical success, though its high budget and promotions costs of an estimated $4 million (equivalent to $70.4 million in 2018), coupled with the lower revenue generated by discounted children's tickets, meant that the film did not make a profit until it was rereleased in the 1940s and in subsequent rereleases. At the 1939 Academy Awards ceremony, Garland received her only Academy Award, a Juvenile Award for her performances in 1939, including The Wizard of Oz and Babes in Arms. Following this recognition, she became one of MGM's most bankable stars.

What was The Wizard of Oz based on ?
a film based on the 1900 children's book by L. Frank Baum.