That would include the Progressive Era, the rise of labor unions after the repression of the 'Gilded Age,' huge immigration from those nasty unwashed peoples from Italy, Sweden, Eastern Europe (lots of Jews, like myn grandparents) and elsewhere pouring into the USA through Ellis Island.
Not everyone came through Ellis Island. If you were Anglo-Saxon, you could come in and settle without the hassle. Like my other grand parents.
This was a time when started to look to the government for solutions to problems with new government entities Pure Food and Drug Act an the FDA, for example and public health, the FTC and the Federal Reserve. This was at the beginning, a reaction against corrupt political bosses and corporations (like the RRs) which make the lives of many Americans difficult. The Progressives were very strong in states such as Oregon and Wisconsin and California.
Previously, the economic and political doctrine was lassez-faire (hands-off), opposing government interference in the economy except to maintain law and order. (Think the modern tea-baggers.) When the change came, it came fast and furiously (think social changes in the 1960s).
Tarriffs were imposed and the fairly new sherman Atni Tris Act was inforced.
During this period was the first income tax (at that time mostly on the very richest, only), prohibition was started, senators were elected directly by voters and women got the vote.
Because of these changes, there was a increase in the middle class. Regular people were able to buy goods such as cars and fine china.
Journalists exposed corruption and scandal in national magazines (a new medium). Science began to be seen as the way to solve problems. Think Thomas Edison & Nikola Tesla.
During the run up to WWI, American did not want to get involved. They elected Woodrow Wilson on a "he kept us out of war' position, but finally sent the American troops 'over there.'
In 1918, during the was, the Great Spanish Flu epidemic. Not Spanish at all, this virilant new strain of flu spread through troop movements all over the United States, to Europe and to the rest of the world and some say that no onein the world was not exposed to the new disease.
This is a really interesting period of American History, probably my favorite. Writers of this period that you should read include Ida Tarbell, Upton Sinclair, Walter Lippman, Booker T. Washinton, herbert Croly, Charles Beard, W.E.B.Du Bois, Lincoln Steffens.
Read "The Jungle." See the movie 'Inherit the Wind.' There's a lot of info out there.