Gao Zhisheng's current whereabouts are unknown after Chinese security agents are said to have rushed to his brother's house, where he is staying, in an isolated village in China's Shaanxi province on Tuesday.
Gao has been under house arrest since 2014 after serving a three-year prison term on subversion-related charges -- a sentence which sparked an international outcry.
"I am worried they will face many threats... I already know that right after (his daughter) Grace's press conference in Hong Kong, Chinese security personnel rushed to his brother's house and threatened (them)," said Bob Fu, president of US-based human rights group China Aid Association which co-published the book.
Speaking in Taipei to launch her father's new book "Stand Up China 2017" -- which predicts the demise of the Communist Party and details his torture at the hands of the authorities -- Grace Gao said her uncle and aunt's mobile phones were disconnected or turned off when she called them on Friday.
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She felt her father would be subject to punishment over the book but added: "He is prepared for anything and our family is prepared."
He was convicted in 2006 of "subversion of state power" and given a three-year suspended prison sentence. State media said in 2011 that he had been ordered to serve the sentence after a Beijing court ruled he had violated the terms of his probation.
In the 446-page book, Gao predicted the demise of the Chinese Communist Party in 2017, saying that "peaceful power for change" will flourish in China despite brutal suppression and it is "enviable for China's evil forces to demise".
The book was published by two human rights groups as no publisher in Taiwan or Hong Kong wanted to get involved, according to co-publisher Taiwan Association for China Human Rights.
"Please help my family and all Chinese people," Grace Gao wrote in a copy of the book to be given to Taiwan's President Tsai Ing-wen through a lawmaker of Tsai's party.
"I hope she will do her best or within her power to help with human rights in China," she said.