BOOK REVIEW : Does Ultra-Orthodoxy Pose Threat to Judaism and Israel? : PIETY & POWER: The World of Jewish Fundamentalism <i> by David Landau</i> ; Hill & Wang $27.50, 358 pages

Can a man truly regard himself as a Jew if he neglects to observe the strict rule against walking between two women, two dogs or two pigs?

The question may seem odd, but only because so many of us--Jew and non-Jew alike--are so ignorant of the body of religious law that continues to govern daily life for the ultra-observant Jew, or “haredi.” And, according to David Landau in “Piety & Power,” the “haredim” are not merely a curiosity--they are openly militant and increasingly powerful in the politics of Israel and the Jewish world.

“The haredim themselves have long felt that the tide of Jewish history is turning their way,” Landau explains. “The isolated pockets of haredism--in New York’s Brooklyn and the Diamond District, Jerusalem’s Mea Shearim--can no longer be dismissed as fading, irrelevant remnants of the pre-Holocaust world. They are bursting at the seams, swelling in numbers, growing in self-confidence and in their determination to influence the world around them.”

“Piety & Power” undertakes two very different tasks, and Landau performs each of them with superior skill and a certain convincing fervor of his own. First, he opens up the world of the haredim--the Hebrew word is equivalent to the phrase “God-fearing”--and shows us scenes of Jewish life that most readers have only encountered in the stories of Isaac Bashevis Singer: the rabbinical court, the yeshiva, the wedding feast, the ritual bath, all cast in the glow of messianism.

At the same time, the book reveals how the haredim have emerged as an aggressive force in Judaism, a Jewish equivalent to the militant fundamentalism that can also be seen in the Islamic world. Both in Israel and the Diaspora, the haredim have shed the mentality (if not the attire) of the ghetto and the shtetl, and they have boldly asserted their right to define the practices of Judaism, to determine the fate of the Jewish state, and, ominously, to decide who is entitled to call himself a Jew.

Landau insists that he “does not argue for or against the haredim,” but “Piety & Power” is so pointed in its characterizations of the ultra-Orthodox that the author’s discomfort and disapproval are obvious enough. Indeed, Landau concludes that the haredim and the values they espouse with mystical intensity are “the time bomb ticking beneath the surface of worldwide Jewish unity and threatening in particular the integrity of American Jewry.”

What will come as a rude shock to most Jewish readers, I suspect, is the conviction among the ultra-Orthodox that Jews who identify with the Reform and Conservative movements are not really Jews at all. But that is hardly the most shocking revelation in “Piety & Power.”

At the ragged edges of the haredim, as Landau shows us, are extremists who blame the Holocaust on secular Zionism: “God’s wrath had been kindled against the Jews because they had sought to recover their Land before His good time.” And one ultra-Orthodox rabbi actually turned to the Palestine Liberation Organization in his struggle to close down a sex shop in Jerusalem:

“The temporary domination of Palestine by the Zionists has caused the desecration and impurity of holy places,” the rabbi wrote to PLO Chairman Yasser Arafat. “We turn to you to help maintain the sanctity of Jerusalem.”

Landau resists the temptation to characterize the haredim as the Jewish equivalent of “Khomeinism,” but he does nothing to discourage us from coming to precisely that conclusion. Indeed, he reports the fact that three haredi members of the Knesset, Israel’s parliament, once demanded that Salman Rushdie’s “The Satanic Verses” be banned “in Israel and everywhere else.”

The ultra-Orthodox communities that are described so intimately in “Piety & Power” may strike some readers as quaint and colorful. And Landau himself writes with compassion about their folkways and their ritual observances. He even recounts a haredi in-joke about one sect of Hasidic Jews that is so strict in enforcing the separation of men and women that even a husband and wife are forbidden to walk together on the street.

“Who was that woman I saw you with last night?”

“Not my wife! Not my wife!”

But Landau, a London-born journalist who lives and works in Israel, does not take his subject lightly, and “Piety & Power” encourages us to look on the “triumphalism” of the ultra-Orthodox as a threat to the Jewish community in general and the state of Israel in particular. No one who conceives of Judaism as a progressive and even permissive faith will come away from Landau’s book with such comfortable assumptions still intact.