For the war itself:
The First World War by John Keegan (one of my favorite authors).
The First World War by Hew Strachan (has the best chapter by far on how the war was financed; dry but valuable)
Cataclysm, the First World War as Political Tragedy by David Stevenson
The Eastern Front 1914-1917 by Norman Stone
Jutland 1916 by Nigel Steel and Peter Hart (if there is one day in history I’d like to go back and see, it would be the Battle of Jutland)
The Road To Verdun by Ian Ousby
A Passage Through Armageddon by W. Bruce Lincoln (On the Russian part in the war, and descent into revolution)
The Doughboys by Gary Mead
1918 by John Toland, which has already been mentioned, but Toland was an excellent historian.
The Defeat of Imperial Germany 1917-1918 by Rod Paschall
The Guns of August was mentioned by many; it is in my library. But it is something of a “standard reference” for the start of the war. The books I listed in my previous post is for those who want to dig a little deeper. Some of Tuchman’s information is dated, but her main conclusions have withstood the test of time.
Thank you for such a comprehensive list!!