The Knight of the White Cross by G. A. Henty

Historical novel. According to the Preface: The order of the Knights of St. John, which for some centuries played a very important part in the great struggle between Christianity and Mahomedanism, was, at its origin, a semi-religious body, its members being, like other monks, bound by vows of obedience, chastity, and poverty, and pledged to minister to the wants of the pilgrims who flocked to the Holy Places, to receive them at their great Hospital -- or guest house -- at Jerusalem, dedicated to St. John the Baptist, and to defend them on their passage to and from the sea, against attack by Moslems. In a comparatively short time the constitution of the order was changed, and the Knights Hospitallers became, like the Templars, a great military Order pledged to defend the Holy Sepulchre, and to war everywhere against the Moslems. The Hospitallers bore a leading share in the struggle which terminated in the triumph of the Moslems, and the capture by them of Jerusalem. According to Wikipedia: George Alfred Henty (8 December 1832 - 16 November 1902), referred to as G. A. Henty, was a prolific English novelist, special correspondent, and Imperialist born in Trumpington, Cambridgeshire, England. He is best known for his historical adventure stories that were popular in the late 19th century. His works include Out on the Pampas (1871), The Young Buglers (1880), With Clive in India (1884) and Wulf the Saxon (1895).