One of the most popular political books this year is getting an update.

Writer John Roughan is adding another chapter to his biography of John Key, 'Portrait of a Prime Minister'.

The revision reveals John Key's thoughts on the 'Dirty Politics' affair.

John Roughan says the Prime Minister appeared unfazed by the hacking and spy allegations that threatened his election campaign.

He notes this was in stark contrast to how ruffled John Key was by the teapot tapes scandal in 2011.

The original version of 'Portrait of a Prime Minister' has sold more than 20,000 copies.