As for the nearby policemen, they embody pure terror in the eyes of Annawadians. They won’t balk at raping a homeless girl, and would “gladly blow their noses in your last piece of bread.” The police actually encourage Fatima to blame Abdul’s family so that officers can extort money from them. A government officer threatens to collect false witness statements unless she is paid off.

Neither India’s hollowed-out democracy nor its mean-minded new capitalism, which cannot do without a helot class, seems able to relieve this social-Darwinist brutishness. Far from being a testimony to the audacity of hope, the dauntless human spirit and that kind of thing, Annawadi turns out to be a gray zone whose atomized residents want nothing more than, in Primo Levi’s words, “to preserve and consolidate” their “established privilege vis-à-vis those without privilege.” Even those who are relatively fortunate, Boo writes, “improved their lots by beggaring the life chances of other poor people.”

Describing this undercity blood sport, “Behind the Beautiful Forevers” (the ironic title is taken from the “Beautiful Forever” advertisements for Italianate floor tiles that hide Annawadi from view) does not descend into a catalog of atrocity — one that a defensive Indian nationalist might dismiss as a drain inspector’s report. The product of prolonged and risky self-exposure to Annawadi, the book’s narrative stitches, with much skillfully unspoken analysis, some carefully researched individual lives. Its considerable literary power is also derived from Boo’s soberly elegant prose, which only occasionally reaches for exuberant neologism (“Glimmerglass Hyatt”) and bright metaphor (“Each evening, they returned down the slum road with gunny sacks of garbage on their backs, like a procession of broken-toothed, profit-minded Santas”).

But “Behind the Beautiful Forevers” is, above all, a moral inquiry in the great tradition of Oscar Lewis and Michael Harrington. As Boo explains in an author’s note, the spectacle of Mumbai’s “profound and juxtaposed inequality” provoked a line of questioning: “What is the infrastructure of opportunity in this society? Whose capabilities are given wing by the market and a government’s economic and social policy? Whose capabilities are squandered? . . . Why don’t more of our unequal societies implode?” Her eye is as shrewdly trained on the essential facts of politics and commerce as on the intimate, the familial and, indeed, the monstrously absurd: the college-going girl who struggles to figure out “Mrs. Dalloway” while her closest friend, about to be forced into an arranged marriage, consumes rat poison, and dies (though not before the doctors attending her extort 5,000 rupees, or $100, from her parents).

You wonder, intermittently, about the book’s omniscient narrator. Perhaps wisely, Boo has absented herself from her narrative. The story of how a white American journalist overcame the suspicion of her subjects (and the outright hostility of the police), or dealt with the many ethical conundrums created by close contact between the first and fourth worlds, belongs to another book. Instead of the faux-naïf explainer or the intrepid adventurer in Asian badlands, you get a reflective sensibility, subtly informing every page with previous experiences of deprivation and striving, and a gentle skepticism about ideological claims.

Boo can see how democracy, routinely lauded in the West as India’s great advantage over authoritarian China, can be turned into yet another insider network of patronage in which the powerful flourish: how periodic elections can be absorbed into “a national game of make-believe, in which many of India’s old problems — poverty, disease, illiteracy, child labor — were being aggressively addressed,” even as “exploitation of the weak by the less weak continued with minimal interference.”

She can also perceive why many well-off Indians have grown impatient with, even contemptuous of, democracy and, like their counterparts elsewhere, want to eliminate rather than enhance the social-welfarist obligations of government. For these Indians, Boo points out, “private security was hired, city water was filtered, private school tuitions were paid. Such choices had evolved over the years into a principle: The best government is the one that gets out of the way.”