To be published after her death, obviously.
Elizabeth II has been monarch for 70 years. He first Prime Minister was Winston Churchill (the standard has declined somewhat since then…). She has met every world leader of any significance since Hitler. She has had privileged access to information about events of huge historical importance for longer than most of us have been alive.
Whether you are a monarchist or not, and I most definitely am not, the Queen is a figure of major historical importance, and it would be interesting for future generations to read her real views about the second Elizabethan era. And it would be fun to read what she really thought of Thatcher, Trump, Brexit etc and how much she really loathes pop concerts…