The U.S. Department of State is the office responsible for issuing passports in the United States. They have a number of dated, but still helpful publications describing other nations' customs, currency, health issues, drug laws, politics, and other things you might consider before you travel abroad.
The Canada Passport Office has an interesting article online about the history of passports. It offers some insightful information about the relationship between Canada and the United States, as it describes the history of the regulation of movement between the countries.
In addition to being a source of information about international history, individual passport applications can be used as a method of finding out more about your family's personal history. The National Archives and Records Administration describes how passport applications can be a tremendous source of genealogical information, and the types of information that they can disclose.