To me this sounds like an extracurricular activity, and a very good one.
I quite like the “applying sideways” blog on the MIT admissions web site, which I believe also applies to getting accepted at other top universities such as Harvard. My understanding of this is to do what is right for you, and do it very well. My family has taken this approach and had some success with it.
I do not think that I would write a book for the purpose of getting accepted to Harvard. I would only write the book if it made sense to me as something that I wanted to do.
But I think that this would indeed stand out if the book reached a “best selling” level of success.