This collection of letters conveniently assembles all of the unpublished correspondence of W. T. Thornton to J. S. Mill and J. E. Cairnes. Although this cache of letters is by no means large, it is nonetheless significant. First, Thornton's letters to Mill and Cairnes provide material for a reassessment of his relationship with both men. Second, it is apparent from these letters that Thornton's intellectual preoccupations ranged widely. Hence, they constitute the best supplement presently available to Thornton's published writings on political economy and philosophy. Third, the letters are an important 'literary' source in recovering aspects of Thornton's illustrious company career at East India House (1836-1880). In this context, they form an invaluable companion to his published writings on India. They may even afford some clues about the role he played in the formation of policy in India in the period after the 1857 mutiny. In short, they cast important light on the social, moral and intellectual milieu in which he lived and worked.