Of course, there must be some assumption that you either have the ability to do extra work that generates income at work (could very well be the case for consul

Of course, there must be some assumption that you either have the ability to do extra work that generates income at work (could very well be the case for consultants), or you can do contract / consulting work yourself. Writing a book falls into the category of being your own boss/business - but there are also uncertainties. The author mentioned that he spent around two months on producing a book, and seeing that he's authored 17 books - that comes out to 34 months over 6 year. But I can't find any information on how much time is being spent during those two months. My estimates were almost two hours / pages or writing a day, and with average of 170 pages a book, that came to 80-90 days of writing, or almost 3 months. So it seems I overestimated.