well his revenue isn't over...most of that is fixed cost...his variable cost is very little.

So a year from now, that same book will still bring in similar revenues(he just has to keep up with the promos).

Meanwhile he can write 4 more books in that same period, and quadruple his income. The next year he can write 5 more, and a year after that he can write another 4.

And altogether they'll make the same income...especially if he builds a reputation for writing good guides, and will begin marketing to his existing customers("hey, if you liked my book on Unix...you'll love my book on Linux")

And so many books are written, because people are hoping to strike it rich. To be the next Harry Potter writer. Same with startups...the stats show that 90% of them fail...but people are still doing it hoping to make billions.