Don't stop at the revenue from the book. There is also the value of credibility from a published technical book. Having this book as a consultant could justify

Don't stop at the revenue from the book. There is also the value of credibility from a published technical book. Having this book as a consultant could justify a high rate for Ruby Unix work. He could gather speaking fees for Ruby and Unix conferences. It could also lead to follow on books from standard publishers. I would say this revenue could be the start to more.