The largest election ever held in history is happening right now in India. More than 800 million people are registered to vote in the world’s most populous democracy. India has been going through dramatic political and social changes since it opened up its economy 20 years ago. It now has a sizable and growing middle class. The transformation has been particularly striking in Delhi, the nation’s capital. Slums were ripped down to make way for posh shopping malls and apartment buildings. In a new book, “Capital: The Eruption of Delhi,” Rana Dasgupta tells the stories of the billionaires, politicians and slum activists bound by the change gripping one of the world’s largest cities.