Hello,
A historian whose books, I liked a lot was the late Sir Martin Gilbert who died in February 2016. Gilbert is best known as Churchill’s official biographer. Gilbert enjoyed the rare honour of having access to Churchill’s private papers since the early 1960’s. This gave Gilbert a clear advantage over other writers.
Presumably as both an established and best selling author, you get access to private papers, unpublished memoirs/diaries which you can use when you are researching your books?
Regards and thanks,
Adrian.