Former US secretary of state Hillary Clinton said Queen Elizabeth's handshake with Sinn Féin's Martin McGuinness in 2012 had been "the most remarkable sign of change yet" in the Northern Ireland peace process.
Writing in her much-anticipated book, Hard Choices, about her time at the state department, Ms Clinton said the handshake was "a gesture that would have been unimaginable just a few years before".
In a chapter on Europe, the former US first lady said that by the time she became secretary of state in 2009 the global financial crisis had "taken a heavy toll on the celebrated 'Celtic Tiger'".
"Roadblocks and barbed wire were gone from the streets but the process of disarmament and 'devolution', which was supposed to grant increasing autonomy to Northern Ireland, was in danger of stalling," she said.
Of the 2009 killings of two British soldiers outside the Massereene barracks in Antrim and a PSNI officer in Armagh, Ms Clinton wrote: “Rather than sparking violence, the murders had the opposite effect.
“Catholics and Protestants marched together in vigils, attended interfaith services and declared with one voice their refusal to go back to the old ways.
“The killings could have been the start of a backward slide. Instead they proved how far Northern Ireland had come.
“They had a chance at lives uncoloured by the pain of the Troubles. I hoped they would never turn back and that their peace and progress would be an inspiration for the rest of Europe and the world.”
Elsewhere, Ms Clinton describes British prime minister David Cameron as "intellectually curious and eager to exchange ideas about world events, from the unfolding Arab Spring to the crisis in Libya and the ongoing debate over economic austerity versus growth".
She described former British prime minister Gordon Brown as "an intelligent and dogged Scotsman" who "ended up presiding over the economic recession that hit Britain hard".
"He was dealt a bad hand, including the baggage of Tony's unpopular support of Bush's decision to invade Iraq, " she wrote.
Ms Clinton said she and British foreign secretary William Hague started off cautiously but she later found him "a thoughtful statesman with good sense and good humour".
