Sometime in the summer of 1918 Calouste Gulbenkian was writing to his son, Nubar, from the Ritz in Paris. The Germans were shelling the city from 75 miles away. Boom! Gulbenkian told his son about a new oilfield in Borneo. Boom! Momentarily distracted when the room shook, he resumed his letter. "One shouldn't allow oneself to be put off by those barbarians and idiots," he wrote. Boom!

That moment at the Ritz perfectly encapsulates Gulbenkian. Nothing distracted him from making money. He was, writes Jonathan Conlin, an "inscrutable force of nature", loyal only to himself. Although he had a British passport, he was, in truth, a citizen of nowhere.

Financial reasons alone caused him to be in France during the First World War. The French, he realised, needed petrol. In 1914 their army had 316 petrol-driven vehicles. Four years later they had nearly 98,000. Thirsty engines caused an acute oil shortage. Gulbenkian sought "to profit from the current situation" and, at the same time, ingratiate himself with the French. From that same suite at the Ritz he also, it seems, provided financial advice to the Turks, France's enemy. That was Gulbenkian.

When he died in 1955 Gulbenkian was the richest man in the world, worth about £5 billion in today's money. His was not, however, a rags-to-riches story. The Gulbenkians, a wealthy Armenian family living in Istanbul, were traders. Calouste, born in 1869, was groomed to take over the family firm, but he had grander ambitions. A few years after his father's death in 1894, he cut his ties with his younger brothers. They went bankrupt as he grew steadily richer.

Gulbenkian's timing was impeccable. In 1897 he made a fortune financing London-based mining syndicates. He dealt briefly with the notorious fraudsters Horatio Bottomley and Whitaker Wright, cutting his ties with them just before the market collapsed and the police arrived. As cars began to appear on European streets, he entered the oil business. His strength, writes Conlin, lay in his "skill at negotiation and his nose for promising deals". He turned his lack of loyalty – to person or country – into an asset. "[His] talent for evading attribution to this or that side would underpin much of his . . . success as a deal-maker."

Gulbenkian wasn't interested in oil, other than in what it could bring. He saw an oilfield only once, at age 19. Despite having business interests in Venezuela, Mexico, the United States and the Far East, he never visited those places, nor did he travel to Iraq, Saudi Arabia or any of the Gulf States, from whose oil production he drew 5 per cent. That dividend came from brokering an agreement in 1928 between the big oil companies to co-operate under the umbrella of his Turkish Petroleum Company to exploit Middle Eastern reserves without wasteful competition. Oil experts at first thought the 5 per cent was simply a broker's commission, but he had cleverly negotiated a permanent share. As one business associate remarked, his calm, dignified method of dealing meant that "he could slip a camel through the eye of a needle".

All that wealth made Gulbenkian a celebrity, although a frustratingly mysterious one. "I should like to know what he really thinks," wrote a society columnist, "whether he has a home; whether he plays golf or has any other interest outside money-making." Gulbenkian provided few clues; he dressed modestly to disguise his wealth. He didn't play golf. He owned palatial homes in Paris and London, but didn't live in them, preferring luxury hotel suites. He was, writes Conlin, a "back room fixer of no fixed abode".

His only interest outside money-making was his art collection. He bought widely – Italian Renaissance, Old Masters, impressionists – but not always wisely. When the Soviet authorities tried to raise revenue by selling off paintings from the Hermitage in Leningrad in the 1920s, one museum official noticed how Gulbenkian seemed possessed by his need to buy. "[He's] the greatest obsessive of them all . . . he kept telling me, 'For God's sake, sell me a painting.' He wanted to buy all our junk." When he was reunited with his collection after hiding it away during the Second World War, he didn't recognise some of his pieces and assumed his minions had sold off the good stuff.

Paintings took on human qualities; he fell in and out of love. One painting "flirted", they mated, then "divorced". His wife, Nevarte, should perhaps have been jealous, but she had her own interests, deriving much more real enjoyment from wealth than he ever did. Their paths hardly crossed. As to her husband's frequent infidelities, she was fatalistic. "This is the way things are . . . we love each other sincerely and if we each close our eyes to the other's faults then . . . we will be very happy." She was probably right; he had no interest in the beautiful women he bedded. His physician had advised that frequent sex with young women was a rejuvenating tonic, so he obediently followed doctor's orders.

Mr Five Per Cent

is a remarkable book, if only because Gulbenkian is not an easy subject. His single-mindedness – in the pursuit of art treasures, sex or money – renders him rather dull. Yet Conlin somehow constructs an engaging tale about this one-dimensional man. Every page is packed with figures, but there are also delightful details that provide welcome contrast to all those labyrinthine deals. An uncharacteristically foolhardy transaction with the Russians, for instance, left Gulbenkian with two tons of caviar and no buyers. He gave the stuff away. Gulbenkian fascinates not because he's particularly interesting in and of himself, but rather because of the shady deals, broken friendships and family turmoil that littered his life.

Gulbenkian, writes Conlin, became "so fixated on protecting his fortune . . . that he seemed uninterested in the purposes for which it was being preserved". That's another way of saying that he worshipped money for itself rather than for what it could do. Other than that brief moment of reflection, Conlin refrains from criticism. Yet this book still provides an important moral lesson about the pathology of greed. We tend to revere those, such as Gulbenkian, who amass huge fortunes. In the process, we overlook their abundant flaws and their lack of ordinary humanity. If Gulbenkian's obsessiveness had been directed towards something other than simply amassing wealth, we might judge him mentally ill.

Gulbenkian's memorial service in 1955 was sparsely attended. At his company headquarters, there was no moment of silence, no condolences extended, no tears shed. That's not surprising. Since he cared about no one, in the end few cared about him.

Mr Five Per Cent: The Many Lives of Calouste Gulbenkian, the World's Richest Man Mr Five Per Cent: The Many Lives of Calouste Gulbenkian, the World's Richest Man