Media playback is unsupported on your device Media caption Xi Nuo: "I'm responsible for this... the booksellers are innocent"

A Chinese writer has told the BBC he is the co-author of a provocative book about China's leader Xi Jinping, that some believe sparked the disappearance of five Hong Kong booksellers.

The US-based writer, who uses the pen name Xi Nuo, says he has published the book online to challenge China and that the men should not be held responsible.

"Why doesn't the government come to New York and sue us?" he said.

The men, who went missing in recent months, are now in detention in China.

Some analysts believe the book their publishing house was set to print, titled Xi Jinping and His Lovers, may have angered the Chinese government and be behind their disappearance and detention.

Media playback is unsupported on your device Media caption Lee Cheuk-Yan: "Whether it's true or not is not really important - people are interested"

Allegations that two of the men were taken to China against their will and in an extra-judicial process sparked global concern. Chinese officials deny this, saying they went to China voluntarily.

Gui Minhai, co-owner of the publishing business Mighty Current, and its affiliated Causeway Bay Bookstore, was last seen at his holiday home in Thailand on 17 October, around the time three of his employees also went missing.

He turned up in early January on Chinese state TV in tears, saying he had returned to China to take responsibility for a fatal car accident that happened more than a decade ago - although some suspect his statement may have been made under duress.

Image caption Gui Minhai showed up on Chinese television in tears about an old drink driving conviction

In late December, Lee Bo, also known as Paul Lee, vanished in Hong Kong, only to turn up mysteriously in mainland China, apparently without his travel document.

His suspected abduction from Hong Kong, a city where mainland Chinese officials have no legal authority, caused an international outcry.

The BBC has also uncovered details of the lucrative and prolific underground world of gossipy pulp books about the Chinese leadership, which caters almost exclusively to a mainland audience, much to the displeasure of Chinese authorities.

Mighty Current publishing house disappearances

1. Lui Bo, General Manager, goes missing in Shenzhen, 15 October

2. Cheung Jiping, business manager, 32, goes missing in Dongguan, 15 October

3. Gui Minhai, co-owner, 51, goes missing in Thailand, 17 October

4. Lam Wingkei, manager, 60, last seen in Hong Kong, 23 October

5. Lee Bo, shareholder, 65, goes missing in Hong Kong, 30 December

Before he disappeared, Lee Bo told me that he believed his colleagues had been detained to prevent the publication of a particularly sensitive book. But some experts say that one book, no matter how incendiary, is unlikely to be the reason for their detention. Industry insiders believe the Hong Kong "banned book" publishing industry is being sent a warning.

Xi Nuo says that his book Xi Jinping and His Lovers was finished in 2014, but Mr Gui decided against publishing it after a visit from a Chinese government agent.

The book also has another author, a man believed to have penned much of the work and who is not being named for safety reasons.

Xi Nuo says he has now decided to publish the book to challenge the Chinese authorities.

Image copyright AFP Image caption Causeway Bay Bookshop carried several titles on Xi Jinping, seen here in a press conference on the disappearances

"I decided to publish this book. I want to tell the Chinese authorities and Xi Jinping, the president of China, that you are wrong. Completely wrong. You better release the five guys. Let them go back home, " Xi Nuo said.

Written in simple and almost vulgar language, the book is ambiguously styled as a work of fiction about true life figures and describes purported affairs of Xi Jinping, as well as alleged incidents in his two marriages.

Excerpt from Xi Jinping and His Lovers

Image copyright Xi Nuo Image caption The front cover of Xi Nuo's book, Xi Jinping and His Lovers

"Outside the door Lingling shouts: 'Big brother Xi please help me, I'm in the kitchen cooking dumplings.' Xi Jinping hurriedly runs out, enters the kitchen and embraces Ke Lingling saying 'My father will be back soon, my father is about to be rehabilitated.' Lingling quickly pushes him away saying 'Aiyo, the way you embraced me, others would tease us.' "

But this work is not unique. A review of the books distributed by Mighty Current reveals many titles that claim to lay bare the private lives of China's top leadership.

They include: Xi Jinping's Clan, Fire in Xi Jinping's Backyard and even The Eight Loves of the General Secretary. Even though many would regard them as poorly researched, the provocative nature of these works has aroused China's ire.

The disappearances are "rooted in efforts by the Communist Party to purge the 'banned books' market in Hong Kong", says veteran journalist and writer Ching Cheong.

A Chinese government directive issued in 2013 and renewed annually, called "Sweeping the source", explicitly targets the Hong Kong publishing industry. It aims to stop "counter-revolutionary"' publishing activities, including preventing such works from entering the mainland.

The books

So, how did these books become so popular? According to Bao Pu, founder of the New Century Press, an independent Hong Kong publishing house, the market for gossipy Chinese political books exploded in 1995.

That was the year Beijing Communist Party chief Chen Xitong was sacked after a major corruption scandal.

Some of the banned books on sale in Hong Kong

Secret Files of Leaders (above): A book that details the alleged affairs of national leaders

Jiang Zemin Wins Big Against Xi Jinping: The book details the alleged rivalry at the very top of Chinese politics between Xi Jinping and Jiang Zemin.

"Before, political books were very serious. But as a result of Chen Xitong, the trend of Chinese political scandals producing quickie books started," says Mr Bao.

The mid-1990s coincided with a rise in overseas travel among newly wealthy mainlanders, who often transited through Hong Kong.

He estimates the market for "quickie" books - poorly researched books written speedily - reached an apex in 2013, with the downfall of the charismatic Chongqing Communist Party chief Bo Xilai providing much material.

He believes 150 books on that subject alone were written in one year, with Mr Gui's publishing house accounting for half of the market.

Image copyright Feng Li Image caption The trial of Bo Xilai is thought to have spawned about 150 books

The money

It is difficult to know the true size of the market but insiders say Mighty Current was one of the top three publishing houses specialising in these books.

Bei Ling, director of the Independent Chinese PEN, a group promoting freedom of expression, says Mighty Current published about five books each month, or 50 books a year, accounting for a third of the market.

Image copyright AFP Image caption People have left messages of support on the door of the bookshop

One writer who has worked with Mr Gui since 2012 said each book takes about a month to write. The writer is not usually paid until the book sells.

For each book sold, he receives a payment of $3, or about 15% of the selling price. On some books, he makes nothing but a bestseller can yield a big payout. The most he has ever made on one book was about $35,000, for several weeks of work. I was also told that the distribution side of the Causeway Bay Bookstore business turned a healthy profit.

The readers

Almost all the readers come from mainland China, hungry for a peek inside China's secretive politics, according to Mr Bao.

Hong Kong is a semi-autonomous Chinese city that enjoys freedom of speech and press. These gossipy books are banned in China, but entirely legal in Hong Kong.

Last year, nearly 46 million Chinese tourists, or more than six times the local population, visited Hong Kong. For some of them, the dingy first-floor bookstore in Causeway Bay is an important stop.

The writers

Mr Gui and his business partner Lee Bo were prolific writers themselves and worked with authors based in Hong Kong, North America and Europe.

Image copyright AFP Image caption Lee Bo's disappearance sparked an international outcry

Many of them are Chinese dissidents living in countries where they are not yet fluent in the language. Writing is seen as a legitimate means of making a living as they adapt to a new environment.

The politics

Many, but not all, of these books criticise the senior Chinese leadership. In fact, one of Lee Bo's recent books on President Xi Jinping was highly regarded and not at all derogatory.

Ching Cheong believes one of the reasons the leadership is so eager to shut down the entire market is that political factions regularly leak information to publishers like Mr Gui to embarrass their rivals.

A faction loyal to former president Jiang Zemin is said to be feuding with the power base of the current president.

Mr Ching says the book market in Hong Kong has become an extension of elite politics. If so, it has become a very dangerous high-stakes game as Mr Gui, who is a Swedish citizen and Mr Lee, who holds a British passport, has transformed it from a "domestic" clampdown to an international affair.

Additional reporting by the BBC's Grace Tsoi.