An excellent and up to date handbook on the burning issue of protecting your digital content. The burgeoning popularity of p2p networks like Morpheus and Kazaa for unauthorised copying and dissemination of audio and [increasingly] video gives urgency to the issue.
The book's chapters fall broadly into two types. One is about encrypting your data so that only authorised customers can access these with some type of key that you furnish.
The other main theme of the book is watermarking or fingerprinting. I use the terms synonymously here. Here, the problem is not so much copying, but for you to later prove that a particular file contains your content. There's some bloody ingenious stuff in these chapters. All about surreptiously embedding a watermark that does not detract from the content's functionality. So that, for example, if it is your audio file, a listener won't notice any degradation. Likewise in imagery. But you can expect a technical opponent to remove your watermark. If it is a JPG file, perhaps he might compress it slightly, and hope this does the removal? The chapters go deeply into this back and forth of tactics.