A World Undone by G. J. Meyer

This is as good a primer for the First World War as you'll encounter. It covers everything from its origins with the Serbs troubling the rump of the Austro-Hungarian Empire (a boil that was lanced with the assassination of Archduke Ferdinand) to its conclusion with the redrawing of the map of Europe. It's particularly good on the political posturing and personalities that led to the whole debacle - military gung-hoism trumped political dialogue. It's brilliant on context. Besides the main action you get potted histories of the Ottoman Empire, the Romanovs, the Hohenzollerns, and the Austro-Hungarian Empire. It also shows, notwithstanding all the guff we read about the Somme, how the Brits stood back from a lot of the engagements and let the French and Russians suffer the cost. The casuality statistics confirm this scenario.