Hong Kong (AFP) - The annual Hong Kong book fair has always been a source of politically sensitive titles banned in China, but this year fewer were on display as the city faces growing pressure under an increasingly assertive Beijing.

Several publishing houses were still displaying controversial books at the harbourfront convention centre as part of a fair that attracts more than a million visitors over six days.

But the number has shrunk since the disappearance of five city booksellers who worked for a publisher specialising in salacious titles about Chinese political leaders.

They vanished almost two years ago and resurfaced on the mainland, where one still remains in custody.

Since then, mainstream bookstores in Hong Kong have removed titles likely to offend Chinese authorities and smaller producers have shied away from rocking the boat.

It comes as many fear that Beijing is tightening it's grip on semi-autonomous Hong Kong and threatening its cherished freedoms.

Greenfield Bookstore has long sold banned works at the book fair and this year displayed titles about late Chinese dissident Liu Xiaobo and former Chinese leader Mao Zedong.

But a store manager who identified himself only as Mr Tam estimated the overall number of politically sensitive books on sale across the fair was down as much as 70 percent.

"There are less of these books coming out. Some of the smaller and independent companies, they haven't come back," he told AFP.

Hong Kong book store owner Paul Tang said that production of gossipy books on Chinese leaders has seen a significant decline since the disappearance of the five booksellers.

Mainland customers who used to travel over the border to snap them up have also stayed away, he said.

"A lot of publishers are self censoring and afraid to produce more books," Tang told AFP.

Flipping through a title about late Nobel laureate Liu at the Greenfield stall, housewife Wincy Chan said she was disappointed the book fair did not address the elephant in the room -- Liu's death last week in Chinese custody.

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"They are avoiding the topic with the theme of travel," said Chan, in her 50s, referring to the fair's focus this year.

Chan said she was worried politically sensitive books would not be available in the future.

But some hope a new wave of writers and satirists will keep the political debate alive.

At the stall belonging to 100 Most, a lifestyle magazine also known for its irreverent take on Hong Kong and Chinese politics, titles included student protest leader Joshua Wong's memoirs and a book about the 2014 pro-democracy Umbrella Movement.

Book-lover Matthew Mok who was browsing the stall said taking a lighthearted approach could be a solution.

"You can't directly talk about things that the Communist Party finds sensitive -- you need a funny way of going into the topic to make the books attractive," Mok, 17, said.