The sea of faith is the Mediterranean in the millennium that we, in the west, call the Middle Ages. In that long and misunderstood era, the lands round what the Romans once called 'mare nostrum - 'our sea' - witnessed an unceasing struggle between Christianity and Islam, two siblings sprung from the same source, over who would inherit the legacy of antiquity. The collision of the two, however different its manifestations, came to fashion a wholly novel Mediterranean and, in so doing, a new world. In a magnificent book Stephen O'Shea evokes the epochal moments of the encounters between the two imperial faiths, bringing to life the instances of their momentous conflict and coexistence. The fall of the Christian Middle East at Yarmuk, Martel's 'wall of ice' at Poitiers, Byzantium's rout at Manzikert, all the way through to Saladin at Jerusalem, Lazar at Kosovo and the suicidal defence of Malta against the Ottomans - these and other fragments of our collective memory come alive and together as part of a huge canvas in this fresh, narrative history. For all those who care about the present day, and the continuing coexistence of these two cultures, history has never been so timely.