Harry made the comments during an interview with trauma expert Dr Gabor Mate.
The one-hour live stream event is happening this weekend.
Bookshops in London opened at midnight to sell copies.
It is ironic that we know so much more about Harry and Meghan since they opted for a more private life writes Peggy Drexler.
"The western occupation of Afghanistan is truly an odious moment in human history."
The Duke of Sussex killed 25 people while fighting in Afghanistan.
The book is on sale in Spain.