Her first book, Math Doesn't Suck: How to Survive Middle School Math without Losing Your Mind or Breaking a Nail, was a New York Times bestseller, and was favorably reviewed by Tara C. Smith, the founder of Iowa Citizens for Science and a professor of epidemiology at the University of Iowa. The book also received a review from Anthony Jones, writing for the School Librarian journal, who described the book as "a trouble-shooting guide to help girls overcome their biggest maths challenges," noting what he described as "real-world examples of great mathematics in action." In an interview with Smith, McKellar said that she wrote the book "to show girls that math is accessible and relevant, and even a little glamorous" and to counteract "damaging social messages telling young girls that math and science aren't for them".
