I first heard of Edward Snowden when I was still a PhD student. He became world-famous for leaking huge numbers of NSA documents on their extensive surveillance and data collection program.
(N.B. To be clear, I do not condone what Snowden did, but even so this book makes for interesting reading.)
How this leak happened, and what the leak contained, is detailed in No Place to Hide by Glenn Greenwald. As Snowden entrusted Greenwald with the leaked documents, Greenwald is in a unique position to offer a detailed and accurate account of Snowden's leaked files.
The bulk of Snowden's documents describe the huge amounts of data are collected from pretty much anyone the NSA can get their hands on, regardless of who they are or where they live in the world. This data comes from various sources: intercepted internet transmissions, agreements with phone companies like AT&T, agreements with tech companies like Google, Facebook, and Microsoft, and other spy agencies. The data collected include the exact times and durations of phone calls, and the contents and metadata of emails. The data accumulated by the NSA is so huge that they have to build massive petabyte data centers to store it.
Greenwald's book is frightening, as he describes his first interactions with Snowden.
Many people, including journalists, denounced Greenwald and his associates for the leaks. A journalist for the New York Times lied in an article, and that was just one of many fabrications Greenwald endured to bring the leaked documents to the world. Greenwald and Snowden also had great support as well. As I said, all of this is frightening. Data-driven technology has only begun to mature and in the future we will see something that we've never seen before.
Greenwald wrote a very interesting book here, reporting on the Snowden leaks. Highly recommended.
By the way, if you're interested in this book, read Bruce Schneier's Data and Goliath. Schneier, an expert in computer security, worked with Greenwald on the Snowden files. Data and Goliath is more focused and detailed on the technical aspects of data collection and espionage, whereas Greenwald's book is more about the initial leak.