That's not in the text of the book, only in the metadata added by Project Gutenberg. You can clearly see her signing her name as M. Curie in the image on that website, and (with a brief Google search) I can't find an example of Curie going by her maiden name after getting married, while there are plenty of examples of the opposite: cf. this letter to Alexander Graham Bell: https://www.loc.gov/resource/magbell.12200101/?sp=1