"Part of the book is about how John Key has cultivated a very respectable image of being friendly and relaxed but at the same time there has been another part of his politics which New Zealanders have not seen or understood."
Speaking to reporters this evening, Mr Hager said the book was about the "unseen side of Mr Key's political management".
He denied he had a political agenda in releasing the book so close to the election.
"Any person, any of you who had access to the documents I have would have leapt at the chance to use this information because it's so illuminating about the Government."
"That's what the job is - to tell people before the election."
He said he got the material early this year and could have taken more time with the book.
"But I've worked like a dog because I believe that people have a total right to know this before the election.
"So have I hurried it out before the election - totally. Is it politically motivated? It's motivated by the public interest that I think people have in knowing what's going on."
Mr Hager said his book would show Mr Key "has some very serious questions to answer".
Many of those questions were about his long term press advisers who were doing things like "going inside the Labour Party's computers", organising other dirty tricks and feeding material to bloggers Mr Slater and David Farrar to orchestrate personal attacks.
"When you read the book you're going to find chapter after chapter of remarkable things that him as leader of the party... he's got a lot to answer for.
"In some cases it's political blackmail, there are many cases of digging sexual dirt to try and scare and threaten people. There are many cases that looked to the country as if they were spontaneous acts of politics which were actually orchestrated from the ninth floor."
Mr Hager described Mr Slater as an "obnoxious" blogger whose attacks on a young man killed in a car crash earlier this year had led to hackers attacking the Whaleoil website.
He said the attack knocked out the Whaleoil site for a number of days during which time a hacker had come away with "thousands and thousands" of documents.
He said the papers revealed an "an astonishingly cynical and ugly view" of the way National's "proxies" behaved in politics.
Mr Hager said people would become increasingly astonished as they read through the book because the "dirty politics" only got worse.
"You will not believe what you read and how bloody awful it is."
In a speech prior to the launch, the book was described as one which "implicates John Key and senior National Party staff".
It ''revolves around a cast of Key, Cameron Slater, Jason Ede, David Farrar, Judith Collins and other National Party figures''.
"It shows a very different side of John Key and his Government than most New Zealanders know'', the book's publishers said.
Hager's earlier books have included Seeds of Distrust, which threatened to ankle-tap former Prime Minister Helen Clark as she headed into the 2002 election.
Mr Hager followed with The Hollow Men which gave voters an inside look at the manipulations of National's 2005 campaign. His 2010 book Other People's Wars exposed New Zealand's intelligence and military activities in Afghanistan and Iraq during the "War of Terror" - a stark contrast to the friendly, public relations-driven view the New Zealand Defence Force had offered.
His previous access to well-placed intelligence sources have driven speculation he was preparing to release Edward Snowden-sourced documents about New Zealand's involvement in the US-led Five Eyes spying alliance. But talk around Wellington yesterday had some leaning towards an expose on political manipulations.
Ahead of today's launch, Prime Minister John Key dismissed Mr Hager saying: "Most people know that Nicky Hager is a screaming left-wing conspiracy theorist."
It's a view which is in contrast to praise from US journalism legend Seymour Hersh, who last year said: "Nicky Hager has more knowledge and understanding of the American intelligence world in Afghanistan - both its good and its very bad points - than any reporter I know."
The release of the latest book has been carried out in total secrecy. Publicity for the book ahead of the launch has been non-existent, although the launch itself at Wellington's Unity Books has become today's must-attend Wellington function.
Media commentator Russell Brown said Mr Key's comments should be seen as preparing the ground for whatever might be coming by attempting to diminish Mr Hager's credibility.
"They are generally stories people don't want told," he said of Mr Hager's work. "You can see that in the push back which comes before the book comes out."
Mr Brown, AUT's Journalist In Residence and frontman with Toi Iti for Maori Television's Media Take, said Mr Hager had proven his credentials repeatedly and should be regarded as credible.
"One very valuable thing about Nicky is that he operates at a different level to most journalists. He has a strong sense in the public interest. He can take a long time to get the detail right. We're lucky to have someone in his position."
Mr Brown said Mr Hager's books fell into either the "barnstormer" or "technical" category and he expected this one would be greeted as a "barnstormer".
Mr Hager's first book was Secret Power, published in 1996. It was a world-exclusive and remains the authoritative text on the United States-led pre-digital spying network called Echelon.
