The Dark Net, a fascinating and disturbing journey through the furthest recesses of the Internet, opens with a story of a young girl who seeks to build her self-esteem by parading naked online via webcam.

Buoyed by positive responses from anonymous viewers of her amateur performance, she is cajoled into posing with her first name written on her skin, and then with a bottle of prescribed medication. With these two acts she inadvertently consigns herself to a vicious humiliation known as a “life ruin”. Within seconds she is traced on social media sites and her friends and family inundated with the naked pictures. Her tormentors post delighted messages of how they reduced her to tears, calling on her mobile phone.

This and other stories in The Dark Net detail a murky world beneath the familiar “surface web” of Google, Facebook and Twitter. Jamie Bartlett, Director of the Centre for the Analysis of Social Media at think tank Demos, is an expert guide. For years he immersed himself in a culture where emails are encrypted, Internet browsing is untraceable, and payments are made using faceless Bitcoin systems.

He bought marijuana from a site called Drugsheaven (it arrived by post in an innocuous white package five days later), and stood in a messy Northern England bedroom while three women earned money performing sex acts for a global audience. Bartlett met neo-Nazis and other political extremists who thrive at the margins of the web.

Yet the book is not intended as an expose so much as an exploration of this underworld. At the heart of The Dark Net is the “crypto war” for privacy in cyberspace - recently thrown into sharp relief by Edward Snowden’s revelations of the Internet snooping programmes of US and UK intelligence services. Bartlett tells the history of the libertarian “cypherpunks” who for 20 years have fought for anonymity and personal liberty on the Internet. At a commune near Barcelona he meets their modern kindred spirits, building an online payment system that could subvert the banks and revolutionise society.

Acknowledging the creativity on the dark web, Bartlett says: “For every destructive sub-culture I examined there are just as many that are positive, helpful and constructive.”

If there’s a criticism of The Dark Net it’s that we see scant evidence of that upside. Even as Bartlett expresses admiration for the vast choice and good customer service on drugs sites such as The Silk Road, he admits their popularity “will tend towards higher levels of use, and drugs use - legal or illegal - creates misery.”

Of 13m page impressions made in one month last year on the anonymous dark web browser Tor Hidden Services, 600,000 were visits to child porn. It’s also hard to admire the technological innovation of the operators of the Assassination Market, which encourages users to compete for financial prizes by guessing the dates when famous people die (thus incentivising murder).