Where can I find information about the history of the United States Postal Service?
Britannica has an article about the USPS that gives some historical information. There are also some books and eBooks available through the library about the post office. Follow the link below to WorldCat and search for su:United States Postal Service.
However, the USPS website appears to be the easiest place to retrieve detailed information about the institution's history. The government publication The United States Postal Service - An American History 1775 - 2006 covers the development of the USPS from colonial times to today.