I don't know how it generally works, but my son-in-law was commissioned to write a book on a very specialist subject by a small, prestigious publisher, as he is an expert in that particular narrow area and no-one has ever properly described it before.It's odd phrasing. I thought books were written under contract, not "commissioned"??? I am not a writer but I have friends who are ... never heard them use this expression.
Happy to be corrected by those who know more ...
My daughter has been contracted to write a book following submission of a proposal to the publishers.