With two separate and successful (by my own standards) ebooks to my name, I inevitably get asked why I don't write an actual book and charge for it. I can't over state how repulsive the thought is to me. There are two reasons for this.
First, I have a love-hate relationship with writing. I know that the day after I'd agree to write a book, I'd regret it. Writing is something I can only do when I don't have to and when there's no expectation of me. Once you agree to get paid you not only have a responsibility to your publisher, but also to the people who are going to spend their money. I know, beyond a shadow of a doubt, that writing and I couldn't survive that sort of pressure.
Let me preface the second reason: this is a generalization which is truer in some circles than others and seems to be getting better (slowly), but... The quality of many books is so poor, I'd be ashamed to be associated with the industry. I'm talking about books that do little more than regurgitate reference material. They are big, uncreative, and generally promote bad programming. Its like MSDN in book form.
Even if a book isn't bad, it'll most certainly be a bad value. We are pioneers of a communication revolution that far eclipses television and radio. We're talking printing press impact. Where radio and televisions were evolutionary by accelerating and enriching the distribution of information, the internet is about redefining the creation of information. Professionally speaking, it shouldn't come as any surprise that we, programmers, are using the internet the way other professions will 10 years from now.
All this is a fancy way of saying that anything you find in a tech book, you can find online..for free. There are things, besides price, that the online version has that a physical book can't beat. First, it's almost always the product of passion (which is true of some books, but certainly not all). Secondly, the online version is organic: it's easier to update and it's often a conversation via comments and linkbacks. Finally, it's easier to search, follow up and share.
I feel like I need to be clear that I'm talking about programming/admin books, not things like like Founders at Work or REWORK. The distinction for me is pretty easy: before charging people, ask yourself can this already be googled?