In 1976 Guest retraced Layard's footsteps to a tribe of "devil worshippers" known as the Yezidis, who had nursed the sick foreigner back to health. He found the descendants of the Yezidis in a remote village in eastern Turkey called Kurukavak. The villagers recalled stories of a strange Englishman who had stayed in their village and were intrigued by his great-nephew.
So began a long association with Turkey. When he provided running water to Kurukavak he became an honoured figure - on arrival he would be lifted on to the shoulders of the locals to cries of "John Guest! John Guest! John Guest!" He had a fountain erected in the village in memory of Layard. Later, back in the United States, Guest traced the strange history of the tribe, who believe in the forgiveness and restoration of the fallen angel Lucifer, in his book The Yezidis, published in 1987 and republished during the Gulf War under the title Survival Among the Kurds.
Guest became a leading scholar in Middle Eastern history and a second book, The Euphrates Expedition, was published in 1992. A further book on the early images of the Middle East, The Ancient Road, will be published later this year.
John Guest was an only child and acquired resilience and independence from an early age. His mother died when he was four years old and, while his father, Lt-Col the Hon Henry Guest, along with three of his brothers, followed their father (the first Viscount Wimborne) into Parliament, John became a King's Scholar at Eton and later graduated BA and MA in Economics from Trinity College, Cambridge.
His innate enthusiasm and energy was caught when young by the promise and informality he found in the US. He first visited in 1933 at the age of 20 and he returned two years later to obtain his MBA from Harvard University. On graduating, he joined the investment banking firm of Kuhn, Loeb & Co in Wall Street, where he remained throughout all the firm's reincarnations, until his retirement in 1989.
Guest brought to the world of finance a clear mind, a prodigious memory and above all a sense of fairness. These qualities secured him challenging tasks and great success. As partner of Kuhn, Loeb & Co and later managing director of Lehman Brothers Kuhn Loeb Inc, his expertise was in transportation. He was instrumental in the restructuring of Penn Central Railroad and was sent to Hong Kong to rescue the failing shipping empire of the future Governor of Hong Kong, Tung Chee-hwa.
During the Second World War, Guest served in the British army in the US, the Middle East and Italy. His linguistic talents were put to use in intelligence work. As a young man he had boarded the Trans-Siberian Railway with no knowledge of Russian and at the end of his journey, one week later, he had taught himself enough to converse easily with his fellow passengers.
John Guest was also an athlete. From his summer house in Fisher's Island, New England, he would begin each day with a bracing swim in the Atlantic. He loved beagling and took up wind-surfing at the age of 70. He once surfed to a friend's yacht for dinner, arriving Bond-like in full black tie.
But, in spite of his energy, charm and success, Guest remained a modest, gentle family man. He married Margaret Houck in 1948 and they had two children; his wife died within days of his death.
John Spencer Churchill Guest, banker, traveller and writer: born London 14 May 1913; married 1948 Margaret Houck (one son, one daughter); died New Canaan, Connecticut 14 May 1997.