After her death in 1997, Morton revealed that Diana was one of his main sources, and that the two worked with the late royal’s friend, Dr. James Colthurst, as a go-between for Morton’s questions and Diana’s answers.
Article source: https://www.huffingtonpost.co.uk/entry/andrew-morton-princess-diana-the-crown-omission_uk_63752731e4b0184e84dde771