Hasidism emerged in 18^th century Eastern Europe as a route to first-class citizenship within a Judaism that had developed a severe hierarchy of legalistic intellectualism. Now, the unlettered Jew could express genuine religious feeling through joy, chant, and the observance of simple commandments.

Hasidism has many strands, but Chabad Lubavitch, named for Lyubavichi, the Russian town where it originated, is the most famous (ironically, as it is the most intellectual of Hasidic philosophies). The reason lies in the response of the sixth Lubavitcher Rebbe, Yosef Yitzhak Schneerson, to the crisis of the Holocaust, which decimated Hasidism along with European Judaism generally. Yosef Yitzhak established a new model of Hasidism, which aspired to persuade all Jews to perform more “mitzvoth.”

This outreach was the first great innovation of Lubavitch, and it was industrialized by Yosef Yitzhak’s successor, the 7^th Lubavitcher Rebbe, Menachem Mendel Schneerson – known worldwide to many, in his lifetime and even 16 years later, simply as “the Rebbe.” The Rebbe sent messengers, known as “schluchim,” to every corner of the planet where Jews could be found. The world’s largest seder now routinely takes place in Kathmandu.

Menachem Mendel Schneerson is the subject of an important new biography by Samuel Heilman and Menachem Friedman, professors at City University and Israel’s Bar Ilan respectively. They describe how Menachem Mendel partially separated himself from Chabad as a Parisian engineer, returning to the fold in flight from the Nazis, shortly afterwards to emerge as Chabad’s undisputed spiritual leader. They also explain Chabad’s oddest mystery. How did this good-hearted organization develop the notion, extraordinarily at odds with fundamental Jewish belief about God’s anointed, that its Rebbe, dead though he was, was the Messiah?

The answer lies in Yosef Yitzhak’s second great innovation: if the Jews were worthy, the Messiah would come – in that generation. The inspiration for this extraordinarily risky strategy (what if he didn’t come?) appears to have been two-fold. First, Yosef Yitzhak had no male children: he was the end of the line. What could be at the end of the line except the Messiah?

Second and more profoundly, as Messrs. Heilman and Friedman persuasively hazard, it was an alternative terminus to that promoted by Hitler. Indeed, the authors plausibly suggest that, saved from destruction by American intervention to preside over an empire from Brooklyn’s 770 Eastern Parkway, now the destination of a pilgrimage by, among others, both American and Israeli politicians and the site of thronging Sabbath crowds, the sixth Rebbe felt guilty that he had accepted a ticket out of Europe while encouraging many of his followers to stay behind.

Risky strategies usually fail, and Yosef Yitzhak died in 1949 with no named successor and no Messiah. It was Menachem Mendel who saved the movement from this double disaster. Devotees of the departed Rebbe will not like the suggestion that Menachem Mendel’s election as successor was in any way a political matter, complete with backroom discussions, but such is the clear if not explicit content of this book. He was clearly the right choice, as his future career showed – but he had to outmaneuver his more conventionally qualified older brother-in-law to prove it.

Second, as for the Messiah, Menachem Mendel simply observed that his father-in-law had not in fact been wrong: Yosef Yitzhak may have assumed incorporeal form, but he was still around. Thus did he defer the problem of the Messiah’s non-arrival — since his harbinger Yosef Yitzhak was still harbingering, as it were.

The seventh Rebbe went on to an astonishing career as Chabad’s spiritual leader, spreading the message to places in distance and number beyond Yosef Yitzhak’s imagination. But Menachem Mendel, too, had a Messiah problem. He was even more invested than his father-in-law had been in the idea that his generation would see Messiah. But neither Messiah nor, more ominously, progeny appeared. Had Menachem Mendel had sons or even sons-in-law, they might have followed his strategy for another generation.

So, as he aged, something at once strange but inevitable happened, like crucial plot developments in great literature. Some Hasidim began to assert that the Rebbe himself was the Messiah — something he neither confirmed nor discouraged. Since the alternative really was now the end of the line for Lubavitch, what other possibility could sustain thousands of young couples who had traveled great distances to places where they were the only observant Jews, eking out “mitzvoth” from their alienated co-religionists?

Not all could stomach an elevation quite so absolute and, when the Rebbe died at 92, the movement split. One segment declared ever more openly that he was the Messiah. The other (without ever saying he was not) simply determined that his activities should be energetically continued.

Remarkably, both groups have sustained themselves thus far, one through sheer persistence of belief, the other by further institutionalizing the Rebbe’s program, expanding “schlichus,” or “messengering,” into “a professional career track for successful Lubavitcher Hasidim, be they young men or women.” Both have also managed to avoid a complete schism, despite a number of law suits.

Heilman and Friedman close with a penetrating analysis of the challenges both groups face in sustaining themselves as the Rebbe’s life and even memory recede.

This is as full and reliable an account of the life of this towering spiritual leader as we are likely to get. The authors will no doubt displease some disciples of the Rebbe, who will not relish their critique of the belief that the Rebbe will return in glory. They would be hardly less offended by the observation that the whole magnificent operation was sustained by the greatest spiritual marketing genius since St. Paul.

But, strange as is the end of the story, no one can or should close this book without knowing that Chabad Lubavitch has, through innumerable institutions large and small, and an extraordinary program of publishing Jewish texts, brought a great deal of spiritual and sometimes material support to Jews everywhere, and that they’ve only begun.

Mr. Rosenberg writes on books and literary culture for the Sun.