Accompanying the BBC series, this book strikes a perfect balance between information and accessibility. Think of it as a beginner's guide to twentieth century warfare; each chapter is not exhaustive but is as good an introduction as you could ask for.
It can be read not just as a book specific to warfare, but one that touches on the development of the world community and international politics in the last hundred years. If you want to learn more on how things have come to be as they are, but are put off by mammoth history books that put you to sleep, give this a try.