I just came back from a trip to Berlin, my first. While there, I found out that during the 1930s and early 1940s, the embassies of some of the major powers (USA, Britain, the USSR) were located on or near Wilhelmstrasse, which meant that they were in close physical proximity to some of the most powerful government departments and ministries of the Third Reich. This got me wondering: did any diplomat from a western country, stationed in Berlin at that time, ever publish their memoirs/diaries/personal recollections of life for Germans at that time? If so, were there any particular publications that became particularly noteworthy.
As I understand it, one of the traditional roles or responsibilities of diplomatic staff is to inform their home governments of social and economic conditions in the countries they are stationed. So I'm assuming someone must have gone to the trouble of publishing a book-length work on the subject, for popular consumption.
Take a look for Andrew Nagorski's book "Hitlerland: American Eyewitnesses to the Nazi Rise to Power." Should be close enough on target, they were Americans, not necessarily diplomats.
Erik Larson's In the Garden of Beasts is pretty on-target, except that it's a historian's retelling of the experiences of William Dodd, U.S. ambassador to Germany, and his family in Hitler's Berlin. It includes pretty extensive quotes and/or paraphrasing of Dodd's personal documents.