The Snow Empress by Laura Joh Rowland
In Japan, 1699. Sa no Ichiro, the samurai detective who has risen to power and influence in the shogun's court, has attracted dangerous rivals. When their son is kidnapped, Sano and his wife, Reiko, set off on a desperate search, only to be trapped by Lord Matsumae, who's been driven mad by the murder of his mistress and is holding an entire province hostage until someone confesses. Sano offers to find the killer if Matsumae will free the hostages. Soon, however, he and Reiko find themselves caught up in an intricate scheme involving racial tensions, jealous husbands, and murderous intrigue.