I've been waiting for this to break.
Please, don't feed the troll.
Let him have his book launch. Don't say a word, save for short, dismissive denials of any mistruths. Pretend that you don't care (because really, you shouldn't; who cares what Garth Turner thinks?). Let media interest peak for a few days and then die off as they move on to some other non-story of the week. If the Tories waste their time engaging in vociferous public denials and criticism of Garth Turner, they will only play to his narcissism and help him sell more books.
Then, six months to a year later, when no one cares anymore, if he has engaged in any legitimate defamation requiring legal response - sue the pants off of him, as quietly as possible. There will be media coverage, obviously, but it will be nowhere near as bad if the Tories come out swinging from the gates and avoiding controversy will ensure that his book launch isn't greeted with massive sales. In short, play it smart don't help the blowhard promote his book.