If you have digital data you want to keep secret, you can encrypt it. But that can just signal the presence of a valuable secret, and encourage people to try to crack the code. Steganography, or 'hidden writing', is a way of hiding your data in other, innocent looking data, so that no-one even suspects it's there. I'd heard of it before, in the context of hiding data in the least significant bits of an image, but this fascinating book told me more about the subject than I had dreamed existed.
Each chapter is broken into three parts: a 'humorous allegory', a general introduction, and the technical meat. I found the allegories sometimes humorous, sometimes bizarre, and sometimes just downright incomprehensible (if there is a hidden message in them, I failed to find it). But the general introductions are excellent: I particularly liked the intuitive explanations of how certain kinds of cryptography work. And the technical meat gives enough detail for people to see how to go about implementing some of the ideas, and pointers into the literature and to the Web for more information.
And for any governments who are worried by steganography, and might be tempted to mandate the use of lossy compression techniques only: don't bother. Someone has already worked out how to hide data in JPEG images!