> Nobody was making money from the process.

From the people I know who wrote or co-wrote books, the way you make money is in future interview processes.

I don't know if they still do it, but when I interviewed for Google, they had a self-ranking system of how competent you are in each technology, and the only way to get the top score was phrased something like "I wrote the book on it (yes, an actual book)".