The standard biography of Mary Cassatt is the excellent volume by art historian Nancy Mowll Mathews, Mary Cassatt: A Life, that I have separately reviewed. I highly commend Cassatt: A Retrospective to all those who enjoy that volume. In the retrospective, you'll find more primary sources, more detail and of course, more examples of Ms. Cassatt's wonderful paintings.
I made the mistake of reviewing the Retrospective volume first . . . and found that it connected the story together not well enough to provide a complete picture. It's like reviewing a magazine issue devoted to an artist rather than a biography. For that reason, I'm commending you to read the books in the opposite order to what I did.
Some of the material in this book is priceless for its ability to add insight into the Parisian artistic scene. I particularly enjoyed reading the reports about the process used to "judge" paintings submitted to the Salon. The relationship between Degas and Cassatt is also clarified by the materials in the Retrospective.