I Will Bear Witness 1942-1945: A Diary of the Nazi Years by Victor Klemperer.
I'll quote from one of the reviews:
...the most disarming and appealing feature of this tome is its slow and ineluctable building of suspense and empathy as World War I veteran Klemperer steadily weaves the day to day details of his life in Nazi Germany in the 12 years of that regime into a portrait of a rogue state moving irresistably down the path to tyranny and terror. The reader is sucked into the vortex of what it is like to live under such circumstances, where an aging Jewish professor who has built a life of purpose and meaning based on scholarship, hard work, and the belief in the rationalism of the state begins to understand that it will all unravel around him.
If you haven't read the books -- it's a 2-book set -- set some time aside this summer. You'll find it unforgettable.