Galileo's Daughter is the story of both Galileo and his wise and intelligent older daughter, Maria Celeste. One hundred and twenty letters from Maria to her famous father survive, and these are the basis of Maria's story.

Because Maria was illegitimate, Galileo put her in a cloistered Franciscan convent outside Florence when she was young. She became a nun and had to live in seclusion, never leaving the convent grounds. Also, since the convent was Franciscan, it had to survive on alms or charity: it could not make its own money and become wealthy, as some other religious communities, such as the Benedictines, did. This made life for Maria and her younger sister precarious, and the nuns sometimes went hungry. This financial situation made it all the more important for Maria to stay in close contact with her well-to-do father, whose donations were much needed.

Maria did what she could for her father. In addition to being an intelligent correspondent, she would sew him clothing, such as collars, and make him candied fruit.

The other prong of the book is the story of Galileo's rise to prominence as Europe's "superstar" scientist, the Albert Einstein of his generation. It focuses in particular on his determination to prove that the earth circled the sun. His observations through his telescope showed him this had to be the case. Nevertheless, it was counter-intuitive, as people could see with their own eyes that the sun seemed to cross the sky in a circle around the earth. Even his own friends thought he was crazy to pursue this reasoning, which butted up against both reason (they thought) and Church teaching. Interestingly, we learn that one of the things that hampered Galileo's construction of a strong argument for his theory was that gravity had not yet been discovered, so Galileo had no framework for expecting a bigger body like the sun to pull the earth into its orbit.

The book outlines Galileo's other scientific theories and his troubles with the Inquisition that led to him having to retract his theory that the earth revolved around the sun.

The book is fascinating reading, in outlining both the lack of power even an intelligent and talented woman like Maria Celeste had over her destiny and the struggles of a scientist in that era to prove his theories.