Plenty of authors and publishers are still concerned about potential cannibalization when book content is also available for free online. This New York Times article, Crossover Dreams: Turning Free Web Work Into Free Book Sales provides even more evidence that free online content can indeed be sold in print. This particular article talks about a book called Diary of a Wimpy Kid and the 147,000 copies it has sold since it was published in April. This, despite that all the content remains freely available at a site called Funbrain.
This is just one successful example, of course, and there are many others. One that I like to cite is our group's very own Naked Conversations, by Robert Scoble and Shel Israel. The book was built on a blog and the content is still there for anyone who wants to read it free online. Despite the fact that the content remains freely available, Naked Conversations is still one of the all-time top-selling books about blogging.