65-year-old Lee Bo, also known as Paul Lee, is a major shareholder of Causeway Bay Books and was the most recent publisher to disappear.
Mr Lee's wife says her husband had called her from a mainland Chinese number to tell her he was safe but would not reveal his location.
Mr Lee's disappearance is the fifth such case related to the bookstore, with four other employees having vanished in mysterious circumstances in the past two months.
The disappearances have fuelled lingering concerns that China is using illegal tactics in the former British colony, whose constitution guarantees rule of law and freedom of expression.
Protest co-organizer Avery Ng, says the fact that Beijing would not confirm or deny its involvement in the disappearances created fear in the public.
"The most scary thing is, there's no news and no proof on what happened to those five publishers, and even the Central Government and the Hong Kong government refused to respond, well previously, on the whereabouts these citizens are. Now if they did not make any arrest, then they just come out and say it. But the fact is, the Hong Kong police and the Chinese police did not respond to those questions. That creates the fear among the Hong Kong citizens."
