This is an astonishing book. It is astonishing at several levels: as a worm’s-eye view of the “undercity” of one of the world’s largest metropolises; as an intensely reported, deeply felt account of the lives, hopes and fears of people traditionally excluded from literate narratives; as a story that truly hasn’t been told before, at least not about India and not by a foreigner. But most of all, astonishing that it exists at all.

Katherine Boo, a Pulitzer Prize-winning American reporter who has worked for The Washington Post, among other publications, spent three years and four months (from November 2007 to March 2011) following the lives of some of Mumbai’s most deprived citizens, the dirt-poor residents of a squatter slum on the periphery of its international airport. Annawadi, in the shadow of luxury hotels, is “a bitty slum popped up in the biggest city of a country that holds one third of the planet’s poor.” It’s built on swampy land and abuts a sewage lake but is home to a motley collection of marginal Indians desperate to make a living out of the detritus of the city’s economic boom. These are the footnotes to the success story of what was briefly called “Shining India,” the poor people who are usually treated as a collectivity, the object of economists’ analyses, politicians’ promises and ideologues’ outrage. In Behind the Beautiful Forevers, Boo humanizes them as individuals with their own stories to tell.

Overcoming the obstacles to effective reporting posed by her class, gender, ethnicity and language, Boo follows their lives and experiences in an effort to understand the problems of poverty from the bottom up. The result is a searing account, in effective and racy prose, that reads like a thrilling novel but packs a punch Sinclair Lewis might have envied.

The narrative teems with larger-than-life figures that Boo instantly draws you to: Abdul, a Muslim teen with a single-minded talent for scavenging recyclable garbage, which “had bestowed on his family an income few residents of Annawadi had ever known”; Asha, who uses political and police connections to climb out of poverty while raising her beautiful daughter, Manju, the slum’s “only college-going girl,” to escape the life of compromises she has led; Fatima, a one-legged neighbor of Abdul’s family, prone to violent rages; Kalu, a boy with the spunk to steal the scrap he then sells to Abdul; and Sunil, a smelly and nerveless ragamuffin with a head for heights. Their stories unfold as Annawadi comes to vivid life, accompanied by a host of lesser but equally indelible characters — the dying man trying to raise money for the operation that might save him, the policewoman seeking to extort money in return for tailoring her case files, the passionate teacher at a juvenile detention center, the young woman who swallows rat poison because “this was one decision about her life she got to make.” And then there’s Abdul’s father, who “had developed an irritating habit of talking about the future as if it were a bus” that one could run after even if one kept missing it. The raw pathos to the stories of the characters in Behind the Beautiful Forevers is of the sort usually found in great fiction, except that, as Boo confirms, they are all real, down to their names.