While some books are static, others (such as reference works and tutorial books) are dynamic in that they need to be refreshed every year or even more often. In the latter case, publishers should think outside the "e-book box" and offer their content as a service.
Taking a cue from the streaming model, publishers can offer continually refreshed books accessible through a browser and charge a subscription, or use an iPhone app as a browser and deliver the app through the App Store (as I have with my "book" Tony's Tips for iPhone Users, which is more like a service). With this model, copy protection is no longer an issue, especially with a $2.99 iPhone app. Just because you can distribute a PDF doesn't mean that's the only way to distribute the content.
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