The Shogun's Daughter

Five months after the earthquake chronicled in The Incense Game, Yanigasawa has found a new way to usurp the Shogun's power.  He's convinced Tokugawa that he's the real father of Yanigasawa's illegitimate son Yoshisato and the Shogun has named the boy his heir.  Yanigasawa uses his new power to demote those unallied to him.  Chamberlain Sano becomes a magistrate in charge of rebuilding while his son Mashihiro ends up as an ordinary page.  With no official duty to investigate crimes, Sino can refuse when the Shogun's wife asks him to find out who killed her stepdaughter but Sano's sense of justice won't allow him to neglect Lady Nobukura's request

The Shogun's Daughter could not inherit on her own, but she could produce an heir. Her death from smallpox led to the succession crisis which allowed Yanigasawa to seize power.  Before Sano and his wife Lady Reiko can determine not only who murdered Tsuriheme - let alone whether she was murdered - he's arrested for the murder of Yoshisato.  There were far too many coincidences for a satisfying puzzle, but the suspense involved in the family's escape made up for that.  There's one one more book in Laura Joh Rowland's series, and I'll be sorry to reach the end.