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Possibly his most important work was Assignment in Utopia an account of his time in the USSR during the late 1920s and early 1930s, which was highly critical of Stalin and his regime, although from within a left-wing standpoint. It received favourable comments from a number of people, including Leon Trotsky and George Orwell, and was an influence on the latter's novel Nineteen Eighty-Four (see two plus two make five).

He also wrote The Red Decade, an examination of CPUSA influence on American cultural life during the 1930s. Ironically it did not prove very popular at the time it was published, since soon after this the Soviet Union became an American ally, but its title became a byword during the era of McCarthyism.

In later years his political views shifted to the right, and for a time he was editor of the Reader's Digest.