The Shogun's daughter / Laura Joh Rowland.

Enhanced descriptions from Syndetics:

Japan, 1704. In an elegant mansion a young woman named Tsuruhime lies on her deathbed, attended by her nurse. Smallpox pustules cover her face. Incense burns, to banish the evil spirits of disease. After Tsuruhime takes her last breath, the old woman watching from the doorway says, "Who's going to tell the Shogun his daughter is dead?"

The death of the Shogun's daughter has immediate consequences on his regime. There will be no grandchild to leave the kingdom. Faced with his own mortality and beset by troubles caused by the recent earthquake, he names as his heir Yoshisato, the seventeen-year-old son he only recently discovered was his. Until five months ago, Yoshisato was raised as the illegitimate son of Yanagisawa, the shogun's favorite advisor. Yanagisawa is also the longtime enemy of Sano Ichiro.

Sano doubts that Yoshisato is really the Shogun's son, believing it's more likely a power-play by Yanagisawa. When Sano learns that Tsuruhime's death may have been a murder, he sets off on a dangerous investigation that leads to more death and destruction as he struggles to keep his pregnant wife, Reiko, and his son safe. Instead, he and his family become the accused. And this time, they may not survive the day.

Laura Joh Rowland's thrilling series set in Feudal Japan is as gripping and entertaining as ever.