He climbed a bit steeper hill than he would have had to climb otherwise because he chose to create something that has lots of free acceptable alternatives already easily findable and useable, and it's (effectively, though not strictly speaking) a commodity product. It might be a pretty darn good one, but it's still something that just fills a commodity need, for most people.

On a related note, I myself have developed a rule of thumb policy to don't buy technical books anymore because there is almost always an acceptable free way to get the info I need, and therefore this frees up more cash for getting things I can't get for free otherwise, or at least, to get higher quality or more convenient things. Like food, shelter, computer equipment, etc. I am still very willing and do frequently buy fiction books, in part because I know how hard it is to write fiction, how poor the pay often is, and because my interest is not a commodity/generic interest but rather a specific one for a particular story or author that is attractive. (I gladly pay for Stephenson, Scalzi and Simmons, for example.) Also, I don't like to handle dirty/secondhand books, and I've read a pretty big percentage of old public domain literature that I'm interested in -- if I haven't read something written before say 1850 there's a pretty good chance I never will because I've passed on it several times already, whereas there are still tons of books written even in the last 20 years that sound attractive from a distance but I still haven't gotten around to reading yet. When I want to Get Something Done I can often do tactical lookups in man pages, help docs, online pages, Googling around, asking somebody, etc. and very rarely have no other recourse than a retail book.

I'm also coming from the perspective of someone who did previously frequently buy technical books and build up a pretty large library of computer/software-related ones, and found that only a small percentage of them were fundamental enough to still be useful and must-keepable today (think Stevens TCP/IP Illustrated, many Addison-Wesley UNIX/Internet tomes). Whereas a lot of them were too tied to transient/temporary/version-specific technology (most of my O'Reilly buys, for example -- even though I love O'Reilly) such as Java 1.2-era books, old versions of Perl, early HTML books, books on Solaris (heck just saying a book on Solaris is antique and useless makes me feel old, I remember when I ran into it at lots of companies before the coming of the Linux wave), etc. Book buying is also a highly discretionary line item in my budget. Whenever there's any sense of a cash crunch or big upcoming need to spend money on more important things, book buying is one of the first things to disappear. Gas prices up 75%? A few less technical books bought and you're back in balance for quite a while.

All this said, I admire writers because I know how hard it is. And I think there will always be a non-zero-dollar market demand for quality written technical books, even if only in purely electronic form. I just think writers will need to increasingly be aware of what their competitors are, because there's going to be an increasing number of good free alternatives to their work, as well. And an increasing percentage of people are going to be giving away portions of their work for free on the web as a sort of resume++ and lead generator.