Complex sentence: 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.
Split into simpler sentences: Possibly his most important work was "Assignment in Utopia". It was highly critical of Stalin and his regime, although from within a left-wing standpoint.