Immigration has always been a hot topic in America, from the time of the first European settlers in the 1500s and 1600s. The U.S. Declaration of Independence lists as one of the grievances against the King his immigration policy. Genealogists are always trying to find the origins of their immigrant ancestors. It is extremely interesting to see how immigration has changed over the centuries. If you would like to understand the background and culture of the area of America covered by the United States, there is one basic book you should read, Fischer, David Hackett. Albion's Seed: Four British Folkways in America. New York: Oxford University Press, 1989.
Ancestry.com has a huge collection of immigration records as do the Castle Garden and Ellis Island Websites.
If you would like to know who came to the United States and when, recent statistics are available from the Office of Homeland Security, but older records are also available from the U.S. Government in two documents;"Historical Census Statistics on the Foreign-born Population of the United States: 1850-1990" and the Bicentennial Edition: Historical Statistics of the United States, Colonial Times to 1970.
There are other resources available and I will highlight them from time to time.