The Sleepwalkers: How Europe Went to War in 1914" by Christopher Clark. I just finished this, which goes into extensive detail about how the politicians in Serbia, Austria-Hungary, Germany, Russia, France and the UK found themselves at war. A heavy focus on diplomacy, and on the Balkans (which makes judging the book by its cover a real mistake, as only one photo predates June 28, 1914, and most are of soldiers and weapons).
As the title says, Clark focuses on "how" - not "why", and eschews making judgments. In that way, Clark makes a very useful addition to the literature. Especially true for anyone with a strong interest in diplomacy, diplomatic history, and the Balkans (like me). Clark makes a strong case for how the Balkans came to be at the forefront of European diplomacy.
It was a not a page-turner (differing it again from Tuchman's Guns of August); Clark's style means that one must commit oneself to reading this serious and long book. It also ends, sharply, on mobilization - this is NOT a war history.
The most evocative passages are found near the end: "Boots", pp. 551-4, contrasting the sorrow of diplomats and ordinary citizens with the "unconcealed joy" of professional soldiers and hawks at the outbreak of war; and (for Kevin), the full paragraph on page 558, distinguishing "the objective factors acting on decision-makers and the stories they told themselves"... "the narratives." (Yeah, I'm not letting that go.)