The USPOD Official Postal Guide was initially published in 1874 and continued in regular publication through the end of 1955. This publication was published regularly in both the monthly and quarterly frequency formats, and was also provided to every post office and postmaster. For most years, there was an annual volume published in January or July, followed by eleven monthly issues, which summarized the significant features which had appeared in the Daily Postal Bulletin.
Downloadable PDF files of the Official Postal Guides are available below. An Introduction which decribes the organization of these files with additional details is available here.
Note: many of these files are quite large, your download times depend upon your internet connection speeds.