Excellent suggestion, Hikaru! The diaries have been published in English as Berlin Diaries, 1940-1945, by Marie Vassiltchikov. They are interesting not only for their portrait of high society life in Hitler's Berlin but also and more importantly for the insight they give us into German resistance against Hitler, which culminated in the July 20, 1944 attempt on Hitler's life.
Marie was actually born in St. Petersburg on January 11, 1917, only a few months before the March Revolution. She grew up as a refugee in Germany, France, and Lithuania, before eventually settling in Germany in the 1930s. She found a job with the Foreign Ministry's Information Department and this is how she first came into contact with Germans in high-ranking positions who were opposed to Hitler, such as Adam von Trott du Solz, a colleague of Count von Stauffenberg. Hers is the only eyewitness diary account of the July 20, 1944 plot that is known to us. I highly recommend this book!