The United States Scott Catalog is far and away the most popular stamp collecting catalog in the world. This comprehensive list contains a ton of information on stamp design, denomination, issuances, and, of course, pricing information. Founded by a man named J. W. Scott, who ran a stamp dealership in New York City during the 1860s, the Scott Catalog claims to list every foreign and U.S. stamp in existence.

Despite recent formatting changes to reduce its bulk, the Scott Catalog weighs in at over 5,000 pages. Of course, despite all its claims, this stamp collecting catalog isn't a ''be all end all'' guide to philately. There's also the Michel Catalog which records oddities, particularly from Middle East nations. Even the assiduous researchers at the Scott Catalog can't discover all of the stamps produced every year.

Indeed, it is estimated that there are almost half a million varieties of stamps in circulation. Millions of plate number series must exist, as well. For advanced help with an obscure corner of philately, you might want to network with in-country experts. Outside philatelists may find it very difficult to penetrate some countries, like Cuba, Haiti, Iran, and North Korea.

That said, while the Scott Catalog may not be a totally comprehensive stamp collecting catalog, you can find great market analyses, advice for stamp collectors, and lists of publishers in its pages. Bear in mind that the Scott Catalog has copyrighted its taxonomy system for displaying stamps. Thus, if you want to publish information about a particular series, you may have check with the Catalog to get its okay.