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.

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.

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.