The Disorderly Knights (Lymond Chronicles: Book 3) by Dorothy Dunnett
Combining all the political intrigue of Game of Thrones with the sweeping romanticism of Outlander, Dorothy Dunnett's legendary Lymond Chronicles have enthralled readers for decades and amassed legions of devoted fans. In this third volume of the series, Francis Crawford of Lymond is dispatched to embattled Malta to assist an order of crusading knights in defending the island against the Turks--only to discover that the greatest threat to the knights may lie within their own ranks.
Having refused a commission from the dowager queen of Scotland, Lymond turns mercenary, heading to Malta to observe the Crusading Order of Knights Hospitaller of St. John, a brotherhood of monks sworn to defend Christendom with swords instead of sermons. The Knights' beloved leader, Sir Graham Reid Malett, is devout and charming, and openly declares it his mission to turn Lymond from his mercenary ways and bring him into the order. But as the Turkish fleet launches a series of devastating attacks, Lymond comes to realize that there may be a much deadlier enemy closer at hand: an adversary who is as subtle as he is savage, and whose piety conceals an absolute genius for evil.