How does having a book for sale affect a writing life?
Not much, to be honest, although it proves that the author can market and sell his books.
When the book sales numbers dwindle from their heights, the author becomes a working writer yet again, but a well-known writer, once he or she publishes again, is an author of another potentially well-selling book. The cycle continues for as long as the author wants it to.
Stephen King, J.K. Rowling and countless others do this.
They create, and book sales result from their efforts.