The two most vocally pro-Beijing newspapers, Ta Kung Pao and Wen Wei Po, are owned by the same government-owned holding company, Guangdong New Culture Development, that owns Sino United.

The market for books on Chinese politics, which have long been a fixture in Hong Kong bookstores that cater to mainland visitors, has fallen on hard times. Interviews with booksellers and publishers say that market forces — rising rents and the advent of e-books — play a part. But so does Mr. Xi’s government, which is increasingly intolerant of any dissent and has warned Chinese tourists that they risk being punished if they return from Hong Kong or Taiwan with banned political books.

During Mr. Xi’s tenure, Sino United has curtailed its purchases of political books, said Bao Pu, publisher at New Century Press in Hong Kong, whose titles include the memoir of Zhao Ziyang, a former Chinese premier. Sales of New Century’s books to Sino United have fallen by 90 percent since Mr. Xi took office, Mr. Bao said in an interview.

That hurts in Hong Kong, with Sino United’s position controlling as much as 70 percent of the market, Mr. Bao said. Last year was the first he failed to make a profit in more than a decade in the publishing industry.

“It’s an existential threat,” he said, adding that he was looking to branch out into graphic novels. “At best we can break even.”

Elvin Lee, an assistant president at Sino United and the chairman of the Hong Kong Publishing Professionals Society, denies that political considerations play a role in the company’s decisions on which books to publish or sell.