Anne Frank's Diary of a Young Girl. It is crushing. Devastating.
She writes of her family's two years in hiding with an other family and an another man, 8 people altogether. During that time she and her writing matures, and she reflects on many serious subjects, such as character (internal and external identity), love, politics, human nature, loneliness, and youth. Throughout the diary are many descriptions of the bickering and difficult dynamics between those in the "Secret Annex". I found these passages deeply moving -- not only was mundane life going on, but you know that underneath the squabbles and tensions was a profound fear. They were all terrified.
Anne Frank's writing is clear as a bell, and she was clearly a talented and precocious teenager.
The diary ends abruptly, as the Annex was raided and all 8 of them were taken to concentration camps. Only Anne's father, Otto, survived. In spite of "knowing the ending", I was shocked and grief-stricken.
Also very intense was the tremendous care the Frank's and Van Daam's received from their helpers. These Dutch citizens were truly heroes.