The Scott Catalog is the bible of philatelists. It is a six or seven volume catalog that describes every postage stamp and related product made in every country of the world since the modern system of postage was started. As could be believed, this is a fairly massive undertaking, and the catalog, across its volumes, runs to thousands of pages of small print and small pictures, much of it densely coded.
It isn't quite clear to me who runs the Scott company, or why they do what they do, except for the fact that philately tends to be a fairly murky, obsessive world. But Scott does have some type of quasi-official information monopoly in the world of stamp selling and buying: the descriptive code that Scott uses for stamps has been adopted by the stamp sellers and buyers for many years, and is so common that if you wanted to buy (for example) a 1956 Postal Card featuring the Statue of Liberty, you could go on ebay and instead of searching around for those terms, just type in "UX 46" and it will pop up easily. This does indeed make things much simpler.
I actually only personally own one volume of the catalog, the "Specialized" volume, which shows only United States and allied stamps, and covers errors and variations in greater detail than in the main volumes. Reading through it is fascinating to me, because of both the catalogs insight into American history, and the feeling of being swaddled in technical details of an obscure, but possibly fascinating world. Be careful before opening this book, before you, too, are stolen into the mysterious world of philately.