The First World War: A Complete History, by Martin Gilbert.
This very long work is essentially a chronology of the Great War, from the rapid escalation of tension before August 1914 to the problems of armistice in 1918 and how they affected state relations in the 1930s. Gilbert, better known as the official biographer of Churchill, manages to humanize the war here. He often makes all too real the nine million military dead of WWI through use of poems, quotes and letters written home by the men who died, as well as graphic recollections by nurses who served at the front (one image that stays with me is a hospital room full of nothing but amputated limbs).
It’s fascinating reading and broad in scope, but it does have its problems. First, the endless litany style does grate after a while. Second, Gilbert is intensely pro-Anglo-American. Thus he ignores all the fighting out of Europe, and while he mentions
In all, it's an impressive feat of scholarship, and tremendously informative for those with little or no grasp of WWI. Its omissions might leave history neophytes with the wrong idea, however, if they aren't scared off by its great length in the first place.