Sinclair's post-World War I period was a combination of an incredible outlay of writing as well as the aforementioned political activisim. In 1927, Oil! was published. Often considered his most effective piece of writing, Oil!, according to critics, illustrated a mark of maturity in Sinclair's writing. Just like the author, the protagonist of the novel rejected the ideals of World War I and was a religious cynic. Oil!, like the majority of Sinclair's fiction, currently is not widely available nor widely read.
During his lifetime, Sinclair received a more generous critical reception abroad than in his homeland. His work was translated into several languages and served, for many Europeans, as an information center about life in the United States. Sir Arthur Conan Doyle, of Sherlock Holmes fame, considered Sinclair one of the world's great novelists. In 1932, Sinclair was a finalist for the Nobel Prize in Literature, although he did not win the award.
Sinclair continued to print most of his novels privately until Viking Press published the Lanny Budd series in the 1940s and 1950s. This series was extremely popular and had a critical following, too. The most respected book in the series was Dragon's Teeth (1942), which chronicled the rise of Nazism in Germany and won the Pulitzer Prize for Fiction. The protagonist in the series — Lanny Budd — is the illegitimate son of a munitions tycoon and witnesses or figures in almost every crucial historical event in a 30-year period, a precursor to the Forrest Gump. Sinclair thought this series of literary history could serve as the texts for school children, but that did not occur.
Sinclair moved to Arizona in 1953 and continued to write. The following year his second wife died. Later that year he married his third wife, Mary Elizabeth Willis. After his wedding, Sinclair published The Autobiography of Upton Sinclair. Willis died in 1967, and on November 25, 1968, Sinclair died. For most of the latter part of the twentieth century, Sinclair was not widely read, primarily because literature with themes of social-change was not regarded as quality literature. Many critics felt that quality literature comments on the human condition but does not explicitly advocate change. That perception may be changing. But even if Upton Sinclair's reputation as an important and significant literary figure does not gain widespread acceptance, The Jungle will undoubtedly remain an American classic.
