After the first Russian revolution in February 1917, the short-lived provisional government appointed a commission to look into the doings of the tsarist regime. The investigators built up a fat file of interviews with people who knew Rasputin, the tsarina's pet guru. Rasputin was already dead, courtesy of two creepy bisexual aristos, a cyanide-laced glass of wine, a Savage .45, a Browning 9mm, an iron bar and the icy waters of the River Neva; but the commission's brief was to establish his character and his presumed influence in the Romanov court. After the Bolsheviks took over, only a few selected highlights of the commission's work were published; and when, during the perestroika period, the popular writer and TV presenter Edvard Radzinsky was allowed to see the commission's archive, he found that the big Rasputin file was missing.

Perhaps some crafty commie had realised its value as a potential nest egg and made off with it. At any rate, in 1995 the conductor Mstislav Rostropovich spotted it in a Sotheby's auction catalogue, entered a successful bid and presented the large bound volume to Radzinsky. The file confirmed Rasputin's debauchery and love of intrigue, but also his magnetism and his religious convictions. This file, together with Rasputin's correspondence, the copious police reports on his daily habits ("Passing time with highly placed ladies has not ended his visits to prostitutes"), various memoirs and a certain amount of conjecture, is the basis of Radzinsky's account.

The "Mad Monk" was neither mad nor a monk. He was a restless Siberian peasant who took to visiting monasteries and discovered a vocation as a lay brother. He arrived in Petrograd in 1903 with a reputation for holiness and a letter of introduction to the bishop. Eventually granted an audience with the tsar, he asked to see the little tsarevich, who was ill and couldn't sleep. As Rasputin prayed by the bed, the boy peacefully nodded off. Miraculous.

The source of Rasputin's power over the royal couple, supposing he had any, was their belief that he could alleviate their son's "haemophilia". The term is supposed to mean a lack of the normal clotting agent in the blood, so that the slightest cut is dangerous. But according to the doctors, Tsarevich Alexei's problem was weak-walled blood vessels. He suffered bouts of internal bleeding, with painful swellings. This condition could be eased by lowering the blood pressure: by simple relaxation, something that a confident, charismatic personality such as Rasputin could well achieve. Radzinsky's theory of "hypnotic suggestion" sounds spookier, but amounts to the same thing.

Rasputin also told the tsarina that Alexei would grow out of the disease, which was what she wanted to hear. Telling her what she wanted to hear was his other great knack. (But he was not her lover, whatever Boney M sang.) In her endless letters to the tsar, putting forward policy changes and candidates for government posts, the tsarina would insist that they were Rasputin's choices and thus God's. But they were usually her own, with Rasputin merely obliged to agree. Radzinsky makes this quite clear, but frequently contradicts himself with claims that Rasputin did indeed hire and fire wartime cabinet ministers and even dictated military strategy, despite the tsar's comment that "Our Friend's opinions about people are sometimes quite odd" and the reality that Rasputin was opposed to war in the first place.

Half the book is devoted to the final two years of Rasputin's life, 1914-16, when his drinking and womanising got out of hand. He believed, like the Khlysty sect, in salvation through sin, because repentance is good for the soul; plus he had a JFK-sized sex drive and a raft of female admirers. The boozing was a long-vanquished addiction that returned under the strain of wartime blues and a growing fear of assassination.

The story drags occasionally, but Radzinsky's analysis of the murder, undermining the version of the conspirators, is shrewd. Rasputin wasn't demoniacally difficult to kill: the killers were just clumsy - over-diluting the poison and then panicking and failing to shoot straight; their arrogant incompetence demonstrated why the Russian nobility had to go. They said they did it to protect the tsar from scandal; but the tsar called them "monsters", and gave up the throne within weeks.

Hugo Barnacle is a novelist and critic. He will be writing regularly for the NS book pages