The Kamo Horse — I.J. Parker

It's the time of the Kamo festival and the imperial horserace. The winner, Wataru, is widely considered the luckiest of men until he finds his beautiful new bride decapitated in his bed. His futile claims of innocence send nobleman-sleuth Akitada hot on the trail of a particularly vengeful killer. (This is a famous tale of Japanese literature recast as a mystery story).