"Diaries and Letters of Harold Nicholson" very good with many behind the scenes peep at key events of the first half of the 1900's.
Moving onto "Portrait of a Marriage" written by his eldest son Nigel. Fascinating insight into how life was for the priviledged, answering the whys and wherefores of Nicholson and, of course, Vita Sackville West (His wife). Some good pictures of Sissinghurst Castle too.