Over the past several decades, the USPS has published a short booklet describing the history of the Post Office in the United States. The USPS identification numbers is Publication 100.
I recently discovered that a new edition of this book has been prepared and is on-line and available for download. This edition is greatly expanded and has a number of color images not found in previous editions. It's a god read, and has many references for further research into the postal system. It can be found here: https://about.usps.com/publications/pub100.pdf