Philately was a semi-obsessive pursuit of mine as a child. As an older teen, with a job, I even gathered some rare items into my collection. Most of what I have of value is of USA origin, though, and I have sent many of the more rare items be authenticated by "experts." Some of the rarities are worth much more after being assessed by so-called experts. Such things as frankings by presidents, postmasters and senators was an interest for a time, but I also became enamored of many engravers who created the art behind the printing of the stamps we take for granted.
I do have some cool stuff from collapsed African nations and many former Brit colonies in proper "complete collection" form and I also maintained a subscription to several postal authorities (many tiny nations and islands, such as Guernsey, Tuvalu, Isle of Man, Sierra Leone, etc.) for new issues of commemorative items, including the US Postal Service. I pretty much ignored them as an adult, and just filed them away when they piled up, but I kept up my subscriptions, buying the yearly issues from USPS, until around '95. My collection of US commemorative issues is complete in MNH condition from the early twenties to around that time.
Some of the interesting (to me) things are more related to the knuckles and elbows of the printing industry, such as sets of color separation proofs and errors where some of the printing is out of registry or even inverted.
If you go to your local library, you can find an issue of Scott's Stamp Catalog and find listings of everything that has ever been minted by the USPS and many foreign nations as well. It is very easy to use and it is considered a retail reference to the value of such common philatelic items. For a more in depth study, I would recommend Minkus, however.
