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

Completely tangential but it's a common thing for a married woman to take the surname of her spouse's family and suppress the maiden name...

Possibily because the most obvious place her name is written is on the image of the front cover, where it's written as "Madame M. Curie". I'd not heard heard her referred to as Marie Sklodowska Curie before - TIL.

Us people are doing that to her name because us people generally use first and last name when referring to individuals and ignore the middle name.

For example:

Melanie Trump

Elizabeth Warren

Steve Jobs

Bill Gates

...

That’s not her middle name. It’s her maiden name.

Because people find it inconvenient to use middle names everywhere, particularly if they’re long?

Skłodowska is her maiden name, not her middle name (which was Salomea).

In American English, when people retain their birth surnames in the middle of their name after marriage, they become "middle names" just like a second given name. Often USAns don't know that this custom is not universal, leading to dreadful faux pas when they assume things like that Hugo Chávez Frías's surname was "Frías".

Didn’t she replace her middle name with her maiden name?