A lot of detail, sometimes hard to keep track of, but also a very coherent theoretical framework for understanding how the various pieces of the European alliance system worked and pushed the continent into war. It's not the story I thought I knew, but it sounds very convincing. Clark emphasises both the remoteness of the world order of the time, and the similarity of many of the motivations to those we see in international relations at the present day.
Despite the title, it doesn't appear that everyone was sleepwalking. Some saw war as a positive development.