Sperry's first book,
One Day with Manu, a colorfully illustrated tale of everyday life in Bora Bora, appeared in 1933. Critic Joan McGrath, cautions modern readers to take his depictions of other cultures in context, stating,
"His early work, such as the tales of Manu, Jambi, and Tuktu, are unlikely to be found in library collections of today, in an era rendered more sensitive to the feelings of minority cultures and racial pride than in the 1930's. Coloured as they were by the prevailing attitudes of his day, Sperry's ethnological works for young readers would by critics of today be stigmatized as condescending in their approach: it is all too easy to lose the historical perspective that would credit him with enlightenment and objectivity, given their date of publication."
Sperry's great-grandfather was a sea captain, inspiring his love of the ocean and his book
All Sail Set about the clipper ship
Flying Cloud, which won him a Newbery Honor Book award in 1936. Although settled in New Canaan, Connecticut, in 1934, Sperry and his family lived Santa Fe, New Mexico for a year, inspiring several books, including
Wagons Westward: The Story of the Old Trail to Santa Fe in 1936 and
Little Eagle, a Navaho Boy in 1938.
On February 13, 1940
Call It Courage was published by The MacMillan Company, the story about a young boy on the island of Hikueru in Polynesia written and illustrated by Sperry. He was awarded the Newbery Medal for 1940 on June 20, 1941 in Cambridge, Massachusetts by the Children's Library Section of the American Library Association. At his acceptance of the Medal, he said, "I had been afraid that perhaps in
Call It Courage, the concept of spiritual courage might be too adult for children, but the reception of this book has reaffirmed a belief I have long held: that children have imagination enough to grasp any idea, and respond to it, if it is put to them honestly and without a patronizing pat on the head."
Sperry purchased a farm in Thetford Center, Vermont in the late 1930s, and then moved to Hanover, New Hampshire at the beginning of World War II. In 1944, he won the New York Herald Tribune Children's Spring Book Festival Award for
Storm Canvas, a story of a boy on the U.S. frigate Thunderbolt in 1814, and in 1949, he won the Boys' Clubs of America Junior Book Award for the 1947 publication of
The Rain Forest.
Although established as a writer, Sperry continued to illustrate dustjackets for other well-known authors of young adult fiction of his era, including Howard Pease, Agnes D. Hewes, Florence C. Means, and Hildegarde Hawthorne, as well as illustrating various basal readers for the Ginn Co. In 1951, he illustrated an adaptation by Allen Chaffee of Longfellow's
Story of Hiawatha.In 1942, he published his only novel for adult readers,
No Brighter Glory, about the Astor family.