Van Gulik's fascination with Judge Dee, an exemplar of the imperial magistrate and of the Confucian scholar, led him to further investigations of Chinese jurisprudence and detection. In 1956 he published his English translation of a thirteenth-century case manual called T'ang-yin pi-shih.

Van Gulik's engrossment with detective literature was soon paralleled by an interest in Chinese erotic literature and art, especially in that of the Ming dynasty (1368-1644). Dalliances with courtesans and concubines were often as much a part of the Chinese gentleman's life as the collecting of ink stones or the playing of the ch'in. To demonstrate this point, Van Gulik, always a connoisseur of Chinese pictorial art, published at Tokyo in 1951 a private edition in fifty copies of erotic color prints of the Ming era along with a handwritten essay on the history of Chinese sex life from 206 B.C. to a.d. 1644. While extramarital sex and the popular novel were generally considered off-limits for the Confucian scholar-gentleman, it is clear that many such men relished illicit sex and enjoyed and wrote novels surreptitiously. Through a number of works Van Gulik showed that although the gentlemen of traditional China often gave lip-service to high moral standards, they displayed in their personal lives the moral weakness of people everywhere,

While the erotica published by Van Gulik circulated only to a select audience, his numerous translations and adaptations of Chinese detective stories made Judge Dee famous in the West, especially during the 1950s. Whether posted in New Delhi, The Hague, or Kuala Lumpur, Van Gulik continued to turn out the Judge Dee stories, to a total of at least seventeen. His final diplomatic appointment brought him back to Tokyo in 1965 as the ambassador of the Netherlands to Japan, a post that he had long coveted. Two years later, while on home leave_, Van Gulik put down his writing-brush for the last time.