Byline: Ivan Watson, CNN
HONG KONG (CNN) -- A government investigation into the mysterious disappearance of five Hong Kong publishers and book sellers this week thrust an unusual publishing industry into the media spotlight.
In this free-wheeling, specially-administered corner of China, several publishing houses and bookshops have spent years churning out books banned on the Chinese mainland.
Speciality bookseller Paul Tang says the books focus on taboo topics: politics, religion and sex. "All forbidden in China," he adds.
Tang spoke to CNN within the tight confines of the People's Bookstore, his book-lined shop and coffee-house perched overlooking Hong Kong's high octane Causeway Bay commercial district.
Cheekily-decorated with vintage posters of Chairman Mao, the shop caters to customers who come almost exclusively from the other side of the northern, "internal" border which divides this coastal city from the rest of China. As for the titles he sells, "50% of our books are not allowed in China."
The covers of many of these books feature portraits of Chinese President Xi Jinping, as well as Bo Xilai, the disgraced Chinese official imprisoned as part of Xi's anti-corruption campaign.
With titles like "Rumor and Truth About Chinese Authorities" and "The Secret Trade Between Rich People and Top Officials," some books peddle poorly-sourced secrets and rumors about the top echelons of China's ruling Communist Party.
Most customers have to hide these banned books when traveling back to mainland China. "They try to hide them in their luggage or handbags ... smuggling," Tang explains.
But despite these precautions, it's not unusual to see customs agents confiscating books from Chinese travelers upon arrival at mainland airports.
Hong Kong's Mighty Current publishing house specializes in sensationalist political books.
"The content of the banned books that Mighty Current publish are mainly gossip and scandals," Tang says. "Very juicy!" he adds with a laugh.
Four of Mighty Current's executives and employees were reported to have gone missing last November.
And on January 1, the wife of Lee Bo, the part-owner of a store owned by Mighty Current, filed a missing person report with Hong Kong police after her husband also disappeared.
In a bizarre turn of events Monday night, Hong Kong police announced Lee's missing person report was canceled upon the request of his wife. She has not responded to multiple requests for comment from CNN.
Lee's store, the Causeway Bay Bookshop, is a second-floor walk-up on another busy street not far from the People's Bookstore.
The store has been shuttered since Lee's mysterious disappearance last week.
Well-wishers have hung handwritten notes next to the "closed" sign on the shop door, with messages like "come home soon and be safe," and "May God bless the staff and family of this bookstore."
"The books here are pretty special," says 26-year old Stephanie Lee, a Hong Kong native and previous customer of Causeway Bay Bookshop. She came with a friend to see the closed store first-hand.
"There's a lack of transparency, no explanation" about the missing publishers, says her friend Michael Lee, a 21-year old resident of Hong Kong. "That's part of what's worrisome."
An editorial in China's state-run Global Times newspaper denounced the Causeway Bay Bookshop on Monday, accusing the store of "causing trouble on the mainland."
In Hong Kong, widespread speculation that the publishers may have been targeted for their criticism of China's Communist Party prompted the city's chief executive to re-assert that "freedom of press and freedom of publication and freedom of expression are protected by law in Hong Kong."
Under the unusual "One country, two systems" arrangement brokered between China and the UK, the former British colony enjoys democratic freedoms denied throughout the rest of mainland China.
And despite the disappearance of the publishers, Hong Kong's forbidden books continue attracting customers from the mainland.
Among those shopping at the People's Bookstore on Monday was a retiree from the southern Chinese city of Guiyang.
The man, who asked not to be named to protect himself from possible retribution in China, said he was looking for books about Chinese history which were censored on the mainland. "I can learn about history here," he says.
Bookshop owner Paul Tang says his family is now worried after the disappearance of other publishers. "I say, I'm just in retail, I'm selling books."
If the authorities tell him to stop, Tang insists, he can easily switch to selling another commodity highly-prized by customers from the Chinese mainland: baby formula.