The French mathematician Sophie Germain was the first woman in the history of mathematics to make a substantial contribution to the proof of Fermat's Last Theorem. Much published research about Germain focuses on her mathematical feats, made under an assumed male name, yet no biography has explained how Germain learned mathematics before that time. Sophie's Diary: A Mathematical Novel is an attempt to answer this question. It chronicles the coming of age of a teenager learning mathematics on her own, growing up during the most turbulent years of the French Revolution. The fictionalised diary uses mathematics and historically accurate accounts of the social chaos that reigned in Paris between 1789 and 1794 to describe Germain's remarkable learning journey. The intellectual and personal struggles of this exceptional young woman will inspire a variety of readers, both students and teachers, mathematicians and novices.