The English author, Lyn MacDonald, has written a series of excellent popular histories of the First World War. They are a good place to learn more about the specific battles of the war or the aspects of the conflict that aren't treated in detail elsewhere. Here are some comments from Penguin:
They Called It Passchendaele, an account of the Passchendaele campaign in 1917.
The Roses of No Man's Land, a chronicle of the war from the neglected viewpoint of the casualties and the medical teams who struggled to save them.
Somme, a history of the legendary and horrifying battle that has haunted the minds of succeeding generations.
1914, a vivid account of the first months of the war. It won the 1987 Yorkshire Post Book of the Year Award.
1914-1918: Voices and Images of the Great War, an illuminating account of the many different aspects of the war.
1915: The Death of Innocence, a brilliant evocation of the year that saw the terrible losses of Aubers Ridge, Loos, Neuve Chapelle, Ypres, and Gallipoli.
To the Last Man: Spring 1918, was published by Viking.
All MacDonald's books are based on the accounts of eyewitnesses and survivors, told in their own words, and cast a unique light on the First World War.