The former Prime Minister says of the SNP leader who he negotiated with on the terms of the 2014 vote: “I always used to say you had to count your fingers on the way out of a meeting with him.”
Mr Cameron says denying a referendum on Scottish independence, it would have been a “much bigger gamble to thwart it” and adds: “I always believed we would win.”
Describing talks ahead of the referendum campaign with the SNP First Minister, Mr Cameron writes: “I went up to Edinburgh for my first negotiation meeting with the slipperiest of characters, Alex Salmond, Scotland's first minister, in February 2012.
“The negotiations dragged on at one point it really seemed the referendum wasn't going to happen, and that his plan was to use the failure to get a deal as yet another argument for independence.
“But by October it was back on and, during a short meeting at Salmond's office in St Andrew's House, we signed the agreement.
“That night I recorded on tape: 'I hope that in 2015 we'll be able to look back and say we saved the United Kingdom.'”
Mr Cameron also describes the awkward moment in 2014 that a bombshell poll putting the Yes campaign in the lead came as he was staying with the Queen at Balmoral.
“Shortly I'd be having an audience with the Queen: she, the woman who had reigned over the UK for 62 years; me, the man who had allowed a vote on its possible disintegration,” he says.
“Of course, she was completely charming the whole household was. But as Prince Philip showed me the barbecue he had designed to roast grouse and sausages up at the hillside bothy, the referendum was clearly on everyone's mind. They gingerly asked questions, but knew they shouldn't express too strong an opinion.
“And then, the next day at breakfast, there it was in cold print. Among the kippers and the kedgeree was The Sunday Times, with the headline "Yes vote leads in Scots poll".
“The Queen wasn't there she usually had breakfast alone. Instead, I was surrounded by ladies-in-waiting, equerries and the moderator of the Church of Scotland. I tried to reassure them about "rogue polls", but I was struggling to convince myself, let alone them.
“One week later, however, the Queen spoke to some of those gathered outside Crathie Kirk and said she hoped Scots would "think very carefully" about the vote. I was delighted.”
Mr Cameron defends the Better Together campaign from accusations that it was ‘Project Fear’, arguing that “if the frame of reference was love for Scotland, that would push undecided voters towards voting yes, not no”
And despite being viewed as a watershed moment in the growth of anti-establishment feeling in British politics, Mr Cameron says the 2014 referendum offers a lesson in “how you deal with the rise of populism”.
“We could easily have ignored the separatism peddled by the SNP. We could have maligned the party and its form of nationalism,” he says.
“Instead, we identified the anti-establishment sentiment early on, confronted it and took the necessary risks.”