I found this book an excellent overview of the latest area of warfare. In an analytical, and still very readable, manner it takes us through the definition of cyber warfare, how it fits into military doctrine (mostly US, but also a bit on other countries) as well as how it is executed (both with logical and physical means). It also covers the legal and ethical views on this type of conflict.
This is not a handbook on the technical details of waging cyber warfare though. While it touches on tools and techniques it does not go into much details on them, which in my view is something good. There are a lot of other books covering the technical parts in much more detail, but very few which puts it all together with the strategic and legal aspects.
Buy this book if you want a concise overview of the field without getting bogged down into details. If you want those it provides you with a good starting point for your further readings.