The disappearance of a Chinese billionaire from his Hong Kong hotel has brought back frightening memories for bookseller Lam Wing-kee, who also went missing in an ordeal that highlighted Beijing's tightening grip on the city.

Lam is one of five Hong Kong publishers who vanished at the end of 2015 and resurfaced across the border in mainland China. He returned to Hong Kong on bail after eight months in detention and refuses to go back.

The booksellers' case sparked international outrage and fuelled concern that Beijing is threatening the semi-autonomous city's freedoms.

Those fears were reignited last month when Chinese tycoon Xiao Jianhua disappeared from his apartment at the Four Seasons hotel, with reports he was snatched by mainland security agents. His whereabouts are still unknown.

Lam says both he and Xiao were political targets.

"It is 100 percent certain the Communist Party is behind it," he said in an interview in Taipei, where he was visiting a book fair.

Lam and the other four booksellers were known for publishing salacious titles about the Chinese leadership and ran a store stocked with books banned over the border.

In Xiao's case, there is widespread speculation that he has been caught up in an anti-corruption drive by Chinese President Xi Jinping, which some critics believe has been a tool to target his political opponents.

The Xiao incident has renewed Lam's fears about remaining in Hong Kong.

"Yes, I worry (about my safety). Every Hong Kong person should worry," the 62-year-old told AFP.

But he added: "I love Hong Kong. I want to stay in Hong Kong. I will never leave. I will speak out even if it means going to jail."

- Enduring trauma -

Lam was allowed back to Hong Kong last June on condition that he pick up a hard disc listing bookstore customers and return to the mainland.

Instead he skipped bail and went public to tell an explosive story of how he was blindfolded by mainland police after crossing the border, and interrogated for months.

Hong Kong has a separate legal system under the "one country, two systems" deal struck when Britain returned the city to China in 1997 and is not obliged to hand Lam back even if he is violating the terms of his bail.

Lam says he became suicidal during his detention and daily life is still not easy.

He wears a face mask to help shield his identity and alternates between the eight entrances to the residential complex where he lives.

While visiting Taiwan last week, he was protected by police security after pro-Beijing protesters attempted to attack prominent Hong Kong democracy activist Joshua Wong during a recent trip to the island.

Lam says he is no longer involved in the book trade, but writes columns for local Hong Kong media on a voluntary basis.

There are still moments when his time in detention comes back to haunt him.

He told AFP he was spooked on spotting a plain-clothes policeman waiting for him at Taipei airport on his arrival.

"I thought he could be a mainland security agent," Lam says.

Three of the other booksellers who were detained have been freed, but Lam says he is not in touch with them out of concern for their safety, believing they are still under surveillance.

The fifth bookseller Gui Minhai -- who disappeared in Thailand -- is still in detention.

But while Lam may have to look over his shoulder for the rest of his life, he says in some ways the trauma of his detention has made it easier to cope with its aftermath.

"When one loses freedom, and then regains it, any problem one faces after that will seem small," he told AFP.

"I just kept thinking about what I went through, which was hundreds, thousands of times worse than this."