Judge Dee (Di Renjie) was a real-life magistrate and statesman of the Tang court, who lived from 630 to 700. He was not a detective (detectives are a modern invention!), but the magistrate of a district, the smallest unit in the Chinese local bureaucracy, which forced him to execute many different duties in own person: head of the administration, head of police, and judge, to name a few (as you see, our modern "separation of powers" didn't exist in ancient China).
Between 1950 and 1968 Van Gulik would write 16 Judge Dee novels. Van Gulik wrote in English, but had the first novel (The Chinese Maze Murders) translated in Japanese by a Japanese friend, and made himself a Chinese translation. The Japanese translation is still available in Japanese bookstores, but as it proved difficult to inspire local detective authors to write about their country's historical heroes, Van Gulik finally resigned himself to writing for an international public in English. That was a good idea. Soon catching on in popularity, the novels were translated into many languages, including Van Gulik's native Dutch (partly by himself).
The first Judge Dee novel I read (a long time ago) was The Chinese Bell Murders, the second one Van Gulik wrote. I was immediately hooked and in high tempo read all the Judge Dee novels the local library had available. After that, I started collecting the missing volumes from second hand bookstores, both in Dutch and in English (at that time, they were out of print in the Netherlands; happily, later new editions appeared).
I was then still in high school, and had already made my decision to study Chinese and Japanese at university. The Judge Dee novels very much strengthened me in that resolve. Reading the novels almost felt as if living in a traditional Chinese city, visiting the market and the temples, the red light district and the Confucius Hall. The books have an original and authentic atmosphere, as nobody knew China better than Van Gulik, who lived there for long periods, was fluent in the language and also wrote many scholarly studies about Chinese culture. In the staunch Confucian Judge Dee, Van Gulik also tried to make us see what the values of educated people in traditional China were, and how their mind worked. We also get fascinating insights into China's material culture, law and punishment, and in human nature in general.
Celebrated Cases of Judge Dee was not translated in full by Van Gulik. He only took the first part, in which Judge Dee solves three cases when he was a local magistrate. And indeed, as a crime novel, that part can stand on its own. In reality, the Chinese original was not a crime novel at all, but a record describing the life of Judge Dee on two levels, first as a loyal servant of the Throne in the provinces, and in the untranslated second part at a high position in the capital, at Court, as a solver of various palace intrigues.
The original Judge Dee novel had one aspect Van Gulik borrowed in most of his own stories: the fact that Judge Dee has to solve several different crimes at the same time, usually three, which Van Gulik considered as more true to life than the single story line in the Western crime novel. But not all aspects of Chinese crime stories were fit for borrowing. Van Gulik rightly skipped such things as that the suspect is known from the start (the emphasis for the Chinese was on crime and retribution, not on suspense and detection) and that the truth is often revealed by supernatural means.
Van Gulik did copy the descriptions of the cruelty of the Chinese police apparatus, where suspects were exposed to severe torture to make them confess (and everyone who entered the magistrate's court was already more or less considered as guilty), although Judge Dee often showed his compassionate side. Van Gulik also included the in China mandatory description of the execution in his own novels (at least in the first five or so). This is also a grisly part (cutting criminals slowly in pieces and things like that), but was necessary in the Chinese context as the stories were after all meant as moralistic admonitions. Happily, there is nothing moralistic about Van Gulik's Judge Dee novels, which are only good fun...
If you have not read Judge Dee yet, I can warmly recommend these novels (both the ones Van Gulik wrote himself and the translation of Celebrated Cases). But be warned, they can be addictive...
Robert van Gulik, Celebrated Cases of Judge Dee (Dover Publications)
Wikipedia article with all titles of the Judge Dee novels.