"economically viable (it is possible to generate a reasonable level of income from it)." The second part of that sentence, is not really the same as the first.

"economically viable (it is possible to generate a reasonable level of income from it)." The second part of that sentence, is not really the same as the first. By his own admission, his book is a real outlier on the high side (second highest in his peer group of O'Reilly books), and it only basically compensated for lost income. This strongly suggests that the median case, is you lose money. Now, there are lots of other great reasons to write a book. The satisfaction of helping others, the space to focus and expand your thinking on a topic, the increase to your reputation, etc. But, "economically viable" doesn't really equate to "at least one or two people didn't lost money doing it".