In 2005, Cullin published his sixth novel, "A Slight Trick of the Mind", a portrait of Sherlock Holmes in old age, for which "The New York Times" praised the author as being "an unusually sophisticated theorist of human nature," and Carolyn See of "The Washington Post" stated that "you don't read it to be 'improved' but for the plain joy of seeing what the language can do in the hands of an affectionate, very accomplished writer." The audiobook edition of the novel won the Audio Publishers Association's 2006 Audie Awards for Unabridged Fiction.