This book impresses me as a homage to the Judge Dee novels by Robert van Gulik - transported to the Japan of the 11th century. Judge Dee has been transformed into Sugawara Akitada, aristocratic scholar-official and sleuth; Ma Joong has become Tora (his enforcer) and Lieutenant Hoong is Seimei (the elder advisor).
The problem is that Akitada is made out to be just such a staunch Confucianist as the Judge Dee of Van Gulik (who is already more Van Gulik than Chinese), and an opponent of Buddhism. That doesn't fit Japan, where a syncretist mixture of Buddhism with the indigenous kami creed (now called Shinto) was the dominant religion in the 11th century - with some admixture of Confucian elements such as filial piety, but none of the strict Confucianism represented here (that came only much later, in the 17th-19th century). The negative view of the Buddhist clergy as criminals is based on Van Gulik's first novel, The Chinese Bell Murders, but was completely alien to 11th c. Japan. Except for a short period when State Shinto was created in the early Meiji period (around 1870) Buddhism was never negatively viewed by the Japanese.
The material culture of the China described by Van Gulik (the 7th-9th c. Tang dynasty seen through the lens of the 14-17th c. Ming dynasty, so already quite complicated!) has become Heian Japan. For example, the aristocracy did sometimes drink tea, as described by Parker, but that was not tea brewed in a pot as she writes, it was brick tea that was ground into a powder and then stirred into a froth in a tea bowl using a whisk (like matcha). The wine house and restaurant culture described in the novels simple didn't exist in Heian Japan, and there were no multistory houses. And "wine" should of course be sake - drunk warm.
Women lived in their rooms and didn't show themselves to men, even not from behind screens - men had to sit outside on the veranda and could not enter (except sometimes secretly at night). Often aristocratic women didn't even speak directly to men, they had their maidservants convey their words. In the novel a sweet potato is offered to someone, but sweet potatoes (satsuma-imo) only came to Japan in the 17th century. Courts of law were not held inside a courtroom as in China, but the magistrate would sit on the floor of the raised building close to the veranda and face the criminals who had to kneel outside in the gravel of the inner courtyard (see for example Kurosawa's film "Rashomon").
I could continue like that, but that would be childish. I.J. Parker was not a Japanologist, but, until retirement, Associate Professor of English and Foreign Languages at Norfolk State University in Virginia. She was interested in Japanese literature and in the Judge Dee novels and that led to the present project, which comprises about 20 novels and several short story collections. That is quite an achievement. She did document herself and the mistakes I mention won't bother an average English reader. "The Dragon Scroll" is quite a lively tale and covers a lot of ground. The detection elements are not very strong, but the novel offers a nice historical panorama. I read it almost in one sitting. I think I'd like to try one of the later novels and see how the author has developed.