APS Logo
You can really advance in your collecting if you join a specialty organization. These are not necessarily huge organizations, but the people who belong are highly dedicated to their hobby. Several of these groups operate sales and auctions. All of them have a newsletter. These groups not only help the newer collector, but provide a venue to the advanced collectors to publish important research and guidebooks. I will try to include contact information in these listings.
American Philatelic Society
Belonging to the American Philatelic Society (APS) is a must for anyone living in the US (and perhaps some folks living in other countries). The group of course is an umbrella for all types of stamp and postal history collecting. You do not need to collect US stamps to benefit from this group! The APS homepage.
Military Postal History Society
The Military Postal History Society (MPHS) is devoted to studying the history of all military mail handling. Their fine book The Postal History of the AEF (currently in update) is full of great information for the collector of military mail from the WW I time period. (The image farther down the page is of military origin, specifically Camp Devens during the First World War.) More about the MPHS.
Western Cover Society
The Western Cover Society studies all postal history relating to the Old West. In recent years, they completed a full listing of all postmarks of California, an activity which spanned several years. It is now being updated. About the Western Cover Society.
Machine Cancel Society
The Machine Cancel Society (MCS) is devoted to studying the history of machine canceling world-wide, including the history of the machines and their makers, as well as the markings themselves on covers and cards. They have their own webpage at: Machine Cancel Society web page. (The image that follows shows the impression of a machine cancel, specifically a Universal machine used at a military camp.)
United States Stamp Society
Washington-Franklin Stamp on Cover
The Washington-Franklin stamp issue (seen above) is just one of the many US stamps studied by members of the United States Stamp Society (formerly, Bureau Issues Association). They try to cover all US issues, primarily from the stamp and production side. However, they have also published many articles about usages of these stamps on cover. They are at the: United States Stamp Society web page.
Mobile Post Office Society
RPO Marking on Cover
The Mobile Post Office Society (MPOS) is a great group to belong to, if you are interested in how mail is handled on moving conveyances. In particular, this group has been aggressive in publishing a catalog of ALL of the RPO (Railway Post Office) routes and cancellations for the United States, and several other countries. Their excellent publishing program has also yielded books on foreign TPO’s, waterways postal markings, and US trolley car post offices [around 1900, many large US cities operated moving post offices on electric and horse-drawn trolley cars!]. About the Mobile Post Office Society.