Pauw's book, The President's Keepers , which exposed alleged shady dealings by Zuma, including tax violations, prompted the State Security Agency to send a cease and desist letter to Pauw and his publishers, NB Publishers.

Exclusive Books CEO Benjamin Trisk said in a statement on Friday afternoon that as South Africa's leading bookseller, the group would continue to stand behind Pauw and his book.

"Part of our task is to sell books that demonstrate and put forward different arguments about matters of importance to the South African public," Trisk said.

"We believe that The President's Keepers is just such a book," he said. Exclusive Books indicated on Twitter that its online bookstore had sold all its copies of The President’s Keepers , and was expecting more stock to arrive on Monday.

Highly compromised and captured state institutions' condemned

The South African Revenue Service (SARS) said on Friday that it was considering taking criminal and civil action against Pauw's book, which was released on Monday.

This was followed by the State Security Agency (SSA), which demanded that the book be recalled and retract certain parts.

Corruption Watch has also weighed in on the debate, saying on Friday afternoon that it condemned the threats made by SARS and the SSA.

"Corruption Watch condemns these actions as those of highly compromised and captured state institutions intent on protecting the president," it said in a statement.

"Corruption Watch fully supports the critical role played by investigative journalists in exposing and combating corruption, which it sees as the major threat to the security of the South African state."

The organisation said SARS and the SSA's attempts to "clamp down on freedom of speech" suggested both parties were prepared to go to great lengths to ensure that Zuma continued to act with impunity and was not held accountable.

"The organisation calls upon the author and his publisher to stand firm in the full knowledge that they enjoy the support of the vast majority of South Africans who labour under the yoke of corruption," it said.

News24