Fall of Giants by ken Follet Book 1 of the Century Trilogy

The novel begins with the thirteen-year-old Billy Williams, nicknamed Billy Twice, going to work his first day in the coal mine underneath the fictional Welsh town of Aberowen in 1911.
Three years later, the main story begins. Earl Teddy "Fitz" Fitzherbert, who maintains a country estate in Aberowen, and licenses the land on which the coal mine is built, hosts a party for many powerful people around the world. His guests include:

• Maud Fitzherbert, Fitz's sister, who is far more liberal than her conservative brother.
• Walter von Ulrich, a German nobleman and a former schoolmate of Fitz's. He and Maud begin at the party to act on the mutual attraction they have felt for years.
• Graf (Count) Robert von Ulrich, Walter's Austrian homosexual cousin.
• Gus Dewar, a highly-educated American who is also a close adviser to President Woodrow Wilson.
• Bea Fitzherbert, Fitz's wife, a Russian Princess.
• King George V, King of the British Empire.
• Mary of Teck, wife of King George V.

Major characters introduced after the party include Gregori and Lev Pushkov, two Russian orphans who work in a munitions factory, and have personal reasons to hold a grudge against Princess Bea and the rest of the Russian royal family.
The characters and their extended families find their fortunes changing for the better and for the worse due to both their interactions with each other and the effects of the First World War.