[ghaff]

You're probably talking quite a few years of on-the-side work to churn out ten books. And, depending upon the type of book, they may only have a shelf-life of 2-3 years.

Also, they're talking about taking home maybe $100-$200/month (which has been about my experience for a tech-related book). $1K/month, to say nothing of $6K/month, to say nothing of this holding up over a period of years, would be very good for a technical book.