I don't know about anyone else, but the reason I buy books is to learn a concept from scratch. Although the internet is a great place to get tips and tricks, I hate clicking through ads and scrolling around to learn a topic. Plus, I'm easily distracted by all these banner ads flashing at me and I sometimes click on them and lose track of what I was originally researching. For me, the biggest appeal of a book is that I can sit down and read an entire chapter relaxing on the couch. No flashy graphics telling me to buy something, no broken links, etc. Just simple text on a white page. I can learn much more from a book than I can from a website. Plus, when I have a problem it's nice to pick up one of my favorite books and flip right to the answer. The biggest benefit of the web is that since most books out there are just copies of the help file, then a Google search can give me answers to the strange problems that no one had the foresight to write about. Nonetheless, the number of books I'm paying for has dropped dramatically and the internet is one of the reasons. Geez, I sound like one of those music industry guys complaining that Napster is the cause for no one buying music anymore. In one sense, this is true. In another sense most books out there are just a new version of the help file so why give them my money? This explains why Wrox went bankrupt last year (Wiley and Apress bought their assets) and other publishers are reporting difficulties.
Since I'm complaining about the quality of books available, let me get sidetracked and talk about one possible reason why: Publishers. When an author signs a book contract the publisher tells the author how much time they have to write the book. From what I've seen, you are given about a week or so to write each chapter. If you were a full-time writer, then this might be feasible. But considering that authors don't make any money off royalties, then not many people can afford to be a full-time writer. So instead you have to work a regular job and spend 8-10+ hours making money. Then you go home at night exhausted and have to crank out a new chapter within a week's time. This is a brutal schedule. Don't forget that you have find time to eat and sleep as well. This is only the first half of the schedule. During the second half you start getting your chapters back from the editors to make revisions. So in addition to having to write new content every week (and stay on schedule) you now have to go back and make changes from the editors. At this point in the schedule you can barely function as a normal human being b/c sleeping just doesn't make it into the schedule. When I worked on my first book I was a walking zombie for the last two months. One guy made a joke in a meeting I was in that "The only time Brian gets to sleep is in our meetings." And it was true. I would fall asleep during the parts that didn't cover the area I was actively working on. So it's no wonder that most books are a copy of the help file. You don't have time to research new topics or explore undiscoverd parts of the software. You just have to crank out more content that sounds good enough to get past the editors. Until authors make enough money from royalties that they can justify spending more time writing a quality book, the internet will continue to be the best source of information.