The Perfumed Sleeve by Laura Joh Rowland

Sano is back and he's got a particularly messy mystery on his hands. A high court official with an complex sexual past and a slew of enimies is dead. The old man was beaten to death and no-one, not his wife who was sleeping next door, his half witted concubine or his handsome houseguest will admit to knowing anything about it. Sano finds himself attempting to deal with the crime while carefully walking a thin line between two opposing factions and the weaked willed Shogun. If he finds the murderer it may ruin his career and life. If he doesn't the country may be plunged into a war.