Determine the type of book you want to write. You could write a book that is a history of the Postal service from its inception to modern day, or you could choose to focus on interesting facts about the post office for younger readers. This could include illustrations of postal workers delivering the mail, to stamps that the post office has issued over the years.
Gather research information about the Postal Service. While secondary source materials can provide a great deal of useful information, it's generally advisable to conduct some primary source research, especially if you are writing a historical work. One site you might want to look at is the National Postal Museum.
Hire an illustrator if you are writing a children's book or if you want to include a drawing on your book's cover or wish to add some drawings throughout your text. Alternatively, you could use public domain images maintained by both the Postal Service and the Library of Congress's American Memories site.
Create an outline for your book. This should include an introduction as well as chapter headings.
Develop summaries for each chapter. You can use note cards to detail each chapter to a degree, as well as add notes or references that will tell you which sources that you have researched and identified. These can be useful as references or for quotes.