"I had no expectation that the Prince would offer me the unprecedented and unfettered access to the original and entirely untapped sources on which this biography is based"

This quote by Jonathan Dimbleby is referring to the access he was given to the original and untapped sources for his biography. It indicates that he marvelled and grateful to get such access from the Prince, as it was unmatched and unanticipated. This access permitted him to develop a more extensive and precise bio than would have been possible without it. It is likely that the Prince had an individual connection to the topic of the bio, and was willing to provide Dimbleby with access to sources that had not been used before. This quote highlights the value of having access to initial and untapped sources in order to produce a more accurate and thorough bio. It also shows the kindness of the Prince in giving Dimbleby such gain access to.