World War I involved the Allies and Central Powers, triggered by the assassination of Archduke Franz Ferdinand. Trench warfare, new technologies, and major battles caused massive loss of life and led to political changes like the Treaty of Versailles and the League of Nations.
You can find books about the aftermath of WWI here.
Grace Banker and Her Hello Girls Answer the Call
Exciting story of the women who went to the front line as Signal Corps operators
Flo of the Somme: The Mercy Dogs of World War 1
Story of a dog’s role in an important WWI battle
Dazzle Ships: World War I and the Art of Confusion
Lively explanation of an innovation used to protect ships during WWI
Dear America: When Christmas Comes Again
Fictional diary of a girl who travels to the Western Front to work as a switchboard operator
The Night Flyers
Story about a girl solving a mystery on the home front while her father serves in WWI
Dear America: Like the Willow Tree
Fictional diary of a girl living through the Spanish Influenza pandemic of 1918
The Sinking of the Lusitania: An Interactive History Adventure
Choose-your-own-adventure stories set on the Lusitania during its destruction
Lord of the Nutcracker Men
Story of a British boy playing with toy soldiers as his dad serves at the front