Recently, I've been glued to the British Diplomatic Oral History Programme website. The BDOHP is collection of transcripts of interviews with former diplomats or other officials who played a significant role in events bearing on British diplomacy, hosted at Churchill College, Cambridge.
There is an absolute abundance of insight and knowledge to be gained from the transcripts. As noted on the BDOHP website, the transcripts "provide valuable details and insights into the daily workings of diplomacy and of Whitehall which may not otherwise be available to outside observers and commentators".
It can take time to digest it all. At times, searching for a particular piece of information is like searching for a needle in a haystack, but there are also unexpected finds that make it all worthwhile.