https://www.theguardian.com/books/2020/may/27/dark-mirror-by-barton-gellman-review-

The author’s account of his interactions with Edward Snowden drive this deeply considered portrait of the 21st-century US-dominated surveillance operation

In January 2013, the documentary film-maker Laura Poitras asked Barton Gellman if he wanted to grab a coffee. The venue was New York. Poitras told Gellman – a former Washington Post reporter – that a few days earlier a mysterious source had been in touch with her.

The person claimed to be from the US spy community. He had news: the NSA or National Security Agency – America’s foremost signals intelligence outfit – had built an unprecedented surveillance machine. It was secretly hoovering up data from hundreds of millions of people. The implications were terrifying. The correspondent said he could supply documents.