I recommend these two books:
Why I Survived the A-Bomb by Akira Kohchi.
Mr. Kohchi was sixteen years old when the bomb dropped. Although he was outside Hiroshima when it happened, he walked towards the mushroom cloud in a dutiful attempt to find his father. This firsthand account has disturbing details.
Nagasaki 1945 by Tatsuichiro Akizuki.
Dr. Akizuki worked at a small hospital at the edge of Nagasaki. Since he did not go to the center of the blast, his account is more detached. During his tireless efforts to treat patients, he realized they showed symptoms consistent with exposure to radioactivity.
These are deeply serious accounts that will leave a lasting impression.