In 2011, there were 620,000 fewer white British people in London than a decade earlier, even as the population rose by roughly the same amount to 8.2 million. They are now less than half the city - 45% compared to 60% in 2001. Some 40% of Londoners were born abroad. More speak little or no English than live in Newcastle.
It is this 'new London' that Judah wants to get under the skin of. Despite peppering his book with figures like the ones above, he opens it with a declaration: 'I have to see things for myself. I don't trust statistics.' True to his word, the book is a journey through the underbelly of London, in the tradition of Jack London and George Orwell. Riding the 4.15am N21 bus from the Old Kent Road to the City with exhausted cleaners before traders have even had their first triple espresso. Listening to the life story of the lovelorn Afghan butcher smuggled in a lorry to Britain and the black White City gangster selling white lines to bankers and billionaires.
It's not just the poor: Judah's 'new London' veers from Neasden to the lonely Russian housewife in Knightsbridge, from the Beckton Alps to the stoned 'Princess' snooped on by her absent sheikh father in Berkeley Square. Baltic tech entrepreneurs and Hungarian software developers on six-figure salaries don't feature. This is a cast of mainly dark, depressed characters on a rain-soaked stage of betting shops, ethnic supermarkets and Belgravia penthouses.
Following Orwell, Judah also goes 'down and out'. He spends a night sleeping with the Roma beggars in the subway beneath Park Lane (who, it turns out, are effectively enslaved to money lenders who loaned them cash for the coach to Britain). He masquerades as a Russian illegal immigrant looking for casual building work, sharing a bed in a Zone 4 'dosshouse' that crams in 14 men. Those statistics don't lie - Ian Gordon, an LSE academic, estimates 55% of the influx of poor migrants to the capital in the 2000s was accommodated by an increase in people per room.
As a narrator, Judah doesn't explicitly judge very often (the occasional interspersions of 'I write this all in my notebook' seem an unnecessary intrusion). But the words of an African policeman (one of a small minority - the Met is 90% white British) seem telling: 'The English are vanishing...London is a patchwork of ghettos.' Blacks are at the bottom of the ethnic pile, the former hotel launderer says, but among the uber-rich 'there is no race'. The migrants are defined by where they came from. And they don't always seem to mix well - two Romanian prostitutes vividly describe how they saw a friend die on the street when she was stabbed by her Pakistani lover.
But to take that as a knife to the heart of race (or British-born and migrant) relations in London would be an overreach. Far from being ghettoised, the capital became less segregated between 2001 and 2011, according to analysis of the most recent census data by Dr Gemma Catney at the University of Liverpool. People of all ethnicities moved out of the city in that same time - the flight was not just white. Indigenous Brits are still the biggest ethnic group in most wards, even as their absolute numbers have fallen.
That's not to say it's all sunshine and rainbows when it comes to integration. Last year UKIP underperformed its general election vote share in most of the capital - save for Barking, home to Judah's 'dosshouse'. But one glaring omission is the riots that burned through much of the city's poorer areas in 2011. That was more a story of poverty than immigration, but then again it is new wealth that is encouraging Eastenders to move to Essex leaving space for poorer migrants in their wake.
There are lighter moments in the book, which is both relentless and on the long side: the Filipina maids holding secret balls in Mayfair mansions while their masters and mistresses are away; the Polish building entrepreneur who owns a house in Balham, a chalet in France and an apartment in Warsaw. The most beautifully bittersweet moment is a Polish registrar alone at a mixed-race wedding party, longing for her Nigerian partner to propose.
Judah notes there are now 405,000 mixed-race people in London. To cherry pick one fine-grained figure, more white-black Caribbean children have been born in the last five years than black Caribbean. Sometimes statistics can be trusted.
This is London: Life and Death in the World City by Ben Judah is published by Picador at £18.99.
Rachel Savage writes for The Economist in Nairobi and was formerly web editor of Management Today. Follow her on Twitter @rachelmsavage