We’re used to the notion that bookstores are quiet, welcoming refuges, not the focus of state-sponsored kidnapping and detentions. But the mysterious disappearances of five booksellers from Hong Kong’s Causeway Bay Books and its eventual closure are reminders of how powerful ideas in books can be.

In 2015, five publishers and booksellers linked to Causeway Bay Books quietly disappeared one by one. One was abducted at a beach resort in Thailand. Two were picked up at their wives’ homes just across the mainland China border from Hong Kong in Shenzhen. One simply vanished.

The kidnapping in Hong Kong of the fifth, Paul Lee, at the end of 2015 left no doubt that Chinese security agents were systematically picking up the owners of the Mighty Current publishing house and its Causeway Bay Books bookstore. In the case of Lee, he was pushed into a minivan when he was delivering books and taken across the border. And then there were none. This was the nightmare scenario many envisioned following the 1997 handover of Hong Kong to China—the scenario in which people exercising their civil liberties within Hong Kong would be snatched.

The Causeway Bay cover-up unraveled when one of those detained, Lam Wing-kee, was allowed by mainland authorities to return to Hong Kong with the promise that he would retrieve information useful to security operatives. Instead, Lam held a press conference, during which he detailed his imprisonment and forced confession.

Until recently, Hong Kong was the freest city in China, and it had long been a beacon of hope in a bleak landscape. The city was home to a lively semi-underground publishing world that specialized in books on China. One of its prominent members was Jin Zhong, who arrived in Hong Kong from the mainland in 1980 and founded a pro-democracy magazine.

“When I came to Hong Kong and found freedoms, I really wanted to enjoy those freedoms,” Jin told me from Brooklyn, where he now lives. “My mission in every article I wrote and in every issue of the magazine was to point out all the wrongs that had been done by Mao and the Communist Party.” The magazine thrived, and Jin set up a publishing house, becoming what was known as a “banned-books publisher,” specializing in books that could not be sold in mainland China.

After the Hong Kong handover in 1997, dissident publishers like Jin played a delicate game with security agents. Visitors from the State Security Bureau would ask him to join them for dinner. It was always cordial, free of coercion, but clearly an offer he could not refuse. The point was to let him know he was being watched.

“The idea wasn’t to make you do or not do something, but to break down your feeling of antagonism or resistance,” Jin recalls. “What happened to the Causeway Bay booksellers—those were the hard methods. For most people in Hong Kong, it was soft treatment.”

Tactics toughened when Jin prepared to publish a book in 2015 by Yu Jie, a Chinese American democracy activist, on Chinese president Xi Jinping. “They made it clear that this book was different,” Jin says. “It was clear that these instructions had come from a high level. If I insisted on going ahead with the book, they would have sabotaged it anyway.”

How could Chinese authorities do that? “They have agents inside the printers and mess up your publishing, cause headaches and massive delays,” Jin explains.

Some years earlier, it had emerged that almost all bookstores in Hong Kong were secretly owned by mainland interests. This infiltration of the book business mirrors the secretive party tactics employed throughout Hong Kong. But Jin has not given up hope.

“Today, Hong Kong’s freedom of the press is being harshly suppressed,” he says, “but I remember what Lin Zhao said during the 1957 Anti-rightist Campaign: ‘We lost in this campaign, but we’re not finished.’ The flame of freedom in Hong Kong has not been completely extinguished; it’s hidden underground, and one day it will burst forth again.”

Free ideas and the books that contain them are a threat to authoritarian regimes everywhere. That’s why Hong Kong authorities now are stripping library shelves of books about Tiananmen Square and Hong Kong’s democracy movement. In an age of instant information and social media, China’s crackdown on booksellers and publishers shows the power of books, how threatening they are to dictators, and how important they are in advancing the cause of freedom.

Mark L. Clifford is the author of Today Hong Kong, Tomorrow the World: What China’s Crackdown Reveals About Its Plans to End Freedom Everywhere. He is president of the Committee for Freedom in Hong Kong.