The late Queen was careful to avoid public interventions in political matters, but the author says she was instinctively against the upheaval of Brexit.
The author quotes royal insiders and an unnamed senior minister, who said the Queen had been frustrated by some of the European Union's bureaucracy, but on balance she thought it was better to remain.
The Queen saw the EU as part of the post-war settlement that she supported, according to the book's sources.
There are other glimpses of the late Queen's dealings with politicians. She got on well with Harold Wilson and John Major but had a much more tense relationship with Margaret Thatcher.
Queen Elizabeth personally rang Tony Blair to congratulate him when the Good Friday Agreement was signed in Northern Ireland in 1998. "I thought, I bet she doesn't do this often," Blair is quoted as saying.
There were other unexpected interventions. She is said to have raised concerns with ministers that defence cuts should not threaten the Army's School of Bagpipe Music and Highland Drumming.