Mr Lam was one of five booksellers who went missing late last year. All were linked to a publishing house specialising in political books critical of China's leaders.
Their case gripped Hong Kong, with allegations that China was interfering with freedom of expression in Hong Kong. One of the men, Gui Minhai, is still in custody.
The BBC's Juliana Liu shares some of the revelations from Mr Lam.
Mr Lam, who owned the Causeway Bay Bookstore before it was purchased by Mr Gui, says he was abducted in the Chinese city of Shenzhen on 24 October on a routine trip to see his girlfriend.
After a night in Shenzhen he was taken, handcuffed and blindfolded, by train to the eastern Chinese city of Ningbo, where he was interrogated irregularly and held until March.
He says he was kept, alone, under 24-hour surveillance and not allowed to leave a small room.
He was not physically abused, but suffered from psychological terror.
The furniture in the room was wrapped in plastic, he believes to prevent any suicide attempts.
"The toothbrush they gave was very small, and connected to a nylon thread," he recalls.
"When you brush your teeth, a guard would hold the other end of the string, and once you finish you have to return the toothbrush. This is because they were afraid I would try to kill myself by swallowing the toothbrush."
"Someone must have done it before."
1. Lui Bo, General Manager, went missing in Shenzhen, 15 October
2. Cheung Jiping, business manager, 32, went missing in Dongguan, 15 October
3. Gui Minhai, co-owner, 51, went missing in Thailand, 17 October
4. Lam Wing-kee, manager, 60, last seen before detention in Hong Kong, 23 October
5. Lee Po, shareholder, 65, went missing in Hong Kong, 30 December
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In March, when three of his colleagues were released and returned to Hong Kong, Mr Lam was transferred to a room in the city of Shaoguan in Guangdong province, where he enjoyed more freedom and served the rest of his detention.
In a twist, on Friday one of his colleagues, British citizen Lee Po, who had been released in March, used Facebook to dispute claims made by Mr Lam that Mr Lee had been kidnapped.
Who exactly was behind the operation to detain all five booksellers, one of whom disappeared from Thailand and another from Hong Kong?
For months, there has been debate on whether the campaign was ordered by the highest levels of the Chinese leadership, or, perhaps, by lower levels of officialdom keen to impress the government in Beijing.
Some believe the crackdown was prompted by the imminent publication of a book about the private life of President Xi Jinping.
Mr Lam says he doesn't know for sure.
But, he says, he was held by officials from the "special investigation team": an ad-hoc cross-agency law enforcement team that can be convened only by the senior leadership in Beijing.
This elite group has roots stretching all the way back to the Cultural Revolution, when its officers were responsible for investigating Communist Party luminaries such as Liu Shaoqi.
More recently, it was believed to be responsible for investigating former security tsar Zhou Yongkang and the former Communist Party chief in Chongqing, Bo Xilai.
Mr Lam says he was released this week on one condition: that he would go to Hong Kong to retrieve a mysterious hard disk filled with the names of people, mainly mainland Chinese, who had purchased books from the Mighty Current publisher.
The bookseller alleges that Mr Lee had provided similar information - with hundreds of names - to the Chinese authorities.
"I now know that they had interrogated me according to those records provided," he says.
"I did not dare tell them about the readers because I was worried they would be implicated, and then they would think that Hong Kong people, I, had betrayed them. But I did not do so."
Mr Lee in his response denied handing over a list of customers to mainland authorities.
Instead of returning to China this week as a snitch, Mr Lam says he decided to skip bail and call a press conference instead to tell his story.
"This isn't just about me. This isn't about a bookstore. This is about everyone. This is the bottom line of the Hong Kong people. This is our bottom line. We will not bow down before brute force," he says.
Hong Kong bookseller Lam Wing-kee, 61, held a defiant press conference on Thursday night to reveal all about his eight-month detention in mainland China.