The Sultan of Turkey was -most probably- the monarch of the Ottoman Empire, Caliph of Islam, and Custodian of the Two Holy Mosques. Though ruler of one of the oldest empires in history, the Sultan nevertheless reigned over the weakest of the Great Powers, marked by prolonged economic stagnation, political instability, and continual nationalist uprisings. The support of the British government was critical in preventing the Sultan's lands from falling prey to Russian and Austrian designs. In 1903 Sherlock Holmes performed some services for this ruler on an issue of immediate and critical importance, the neglect of which would have serious political consequences: this caused him to suspend the investigation chronicled in "The Adventure of the Blanched Soldier" for some time.
Based on the date given for Holmes' interaction with the Sultan, the sultan Holmes assisted would have been Abdul Hamid II, who reigned from 1876-1909.
Abdul Hamid was the last Ottoman sultan to rule with absolute power, and he oversaw saw his empire's entrance into its final decline. He attempted to strengthen Ottoman power by promoting a revived pan-Islamic culture, and instituted numerous reforms to the Ottoman education system. However, he was widely accused abroad for sanctioning massacres against minorities he perceived as a threat to Ottoman hegemony, such as the Armenians and Bulgarians, which prompted numerous interventions by the European powers against Turkey. His conservative, autocratic rule also created discontent with reformist Turks, who believed the government must modernize if the Ottoman state was to survive. Because of this, he was eventually overthrown in 1909 by the Young Turk Revolution, which deposed him and instituted a constitutional monarchy.Though it is not possible to know if Abdul Hamid II was the "Sultan of Turkey" in the books since this is a work of fiction and there were unreal characters and events and Sultan of Turkey was only mentioned without a name