The Ringed Castle (The Lymond Chronicles #5) by Dorothy Dunnett

Fifth in the legendary Lymond Chronicles, The Ringed Castle leaps from Mary Tudor's England to the barbaric Russia of Ivan the Terrible. Francis Crawford of Lymond moves to Muscovy, where he becomes advisor and general to the half-mad tsar. Yet even as Lymond tries to civilize a court that is still frozen in the attitudes of the Middle Ages, forces in England conspire to enlist this infinitely useful man in their own schemes.