Jung Chang and Jon Halliday have performed an amazing feat in tracking down and assembling vast amounts of detailed information about one of the most destructive historical figures of all time. They interviewed hundreds of people – relatives, friends, colleagues, families of colleagues, staff members, witnesses to historical events, and diplomats and politicians from dozens of countries who met Mao. They consulted scores of archives in 10 countries and made use of vast numbers of both Chinese and non-Chinese language sources. It is hard to imagine a more thoroughly researched biography.

“Mao: The Unknown Story” (Alfred A. Knopf, 810 pages, $35) brings to light many little-known facts about recent Chinese history and the reign of Mao Zedong. For example, in 1948, during the civil war, civilians were barred from escaping the city of Changchun, which was besieged by the communist forces. Mao believed that the defenders “could be pressured into surrendering by massive civilian deaths … By the end of the five months siege the civilian population had dropped from half-a-million to 170,000.The death toll was higher than the highest estimate for the Japanese massacre in Nanking in 1937.”

Or, during the famine caused by the Great Leap Forward (1958-60), urban housewives were getting 1,200 calories of food a day. Slave laborers at Auschwitz used to get between 1,300-1,700. The average daily calorie intake in China in 1960 was 1,534, according to official statistics. There were many documented incidents of cannibalism, and altogether 38 million people died of “starvation and overwork” in that period.

Mao belongs to the small, select group of 20th-century totalitarian dictators (with Hitler, Stalin, Kim Il Sung, Pol Pot) who were consumed by the aspiration to radically transform their society and their fellow human beings. They were subjects of grotesque, quasi-religious cults and accumulated vast amounts of concentrated power. They were unscrupulous, ruthless, devious, duplicitous, cunning, paranoid, and megalomaniacal.

There was a congruence between their personalities and the political systems they helped to establish and presided over. With the exception of Hitler, they were astonishingly successful in creating a durable illusion of their benevolence and their devotion to the welfare of their people. Stalin and Mao in particular captivated innumerable Western public figures and intellectuals. There has never been such a contrast between true personality and public image.

Mao and Stalin shared a sense of mission and a calculating ruthlessness, going to great lengths to eliminate potential rivals. Each encouraged his own deification and used terror to solidify his power. In 1951, Mao “issued order after order berating provincial cadres for being too soft and urged more ‘massive arrests, massive killings.'” He expressed “delight” when one province “raised its execution rate.” Mao, unlike Stalin, favored public executions (as well as public humiliation and torture), which he felt maximized their impact.

The totalitarian characteristics of Chinese communism emerged as early as the 1930s in areas under Mao’s control: In the words of Ms. Chang and Mr. Halliday, “China’s first red state was run by terror and guarded like a prison … In this prison-like universe, suicide was common – an early wave of what was later to grow to a flood throughout Mao’s reign.” The totalitarian tendencies were also apparent (while still fighting the Nationalists) when he introduced a Social Relationship form for those under his control to describe their “every single social relationship.”

Where Mao differed from his fellow totalitarian tyrants was in his lechery and pursuit of luxury. The possession of power was not enough for him. While he imposed ultra-puritanism on his people, he preyed on great numbers of young women (“imperial concubines”) procured matter-of-factly by his underlings. Mao also had high expectations of comfort. This was already manifest in Yenan, when he made the single car available to his forces – donated by Chinese laundry workers in New York “for carrying war wounded” – his private vehicle. “People like me only have a duty to ourselves,” he wrote. When he gained power over the entire nation, his lifestyle rivaled that of any Chinese royalty. During his 27-year rule, more than 50 huge estates were created for him in different parts of the country.

Mao’s devotion to the welfare of his people was reflected in his allocating 61% of the budget to the military and arms production and 8.2% to education, culture, and health combined. In 1957 he avowed being ready “to sacrifice 300 million Chinese for the victory of world revolution” – at the time, half of the population. His obsessive pursuit of superpower status led to wasteful and ludicrous projects such as the backyard furnaces in villages that were supposed to increase steel production but instead diverted labor from agricultural work and resulted in the melting down of useful implements and machinery.

Ms. Chang and Mr. Halliday show that, contrary to widespread impressions, the Cultural Revolution was not spontaneous but a state-sponsored purge instigated by Mao that killed some 3 million people. Many readers may find it hard to believe that “Mao controlled when and how his Politburo members could receive [medical] treatment” or vetoed treatment for Zhou Enlai’s cancer in 1972. Such calculated cruelty is shocking, but it is dwarfed by the damage Mao inflicted on tens of millions of ordinary Chinese. The authors conclude that “well over 70 million people perished … as a result of his misrule.” This puts Mao at the top of the list of ideologically inspired mass murderers of all time.

Readers of this powerful and absorbing book will wonder what made it possible for a repellent human being such as Mao to seize and keep absolute power for decades and – even more puzzling – how he succeeded in impressing many Westerners about his benevolence and omniscience.

Mr. Hollander is author of numerous books about communist systems and their Western perceptions. He is the editor of “From the Gulag to the Killing Fields: Personal Accounts of Political Violence and Repression in Communist States,” which will be published by ISI in November.