Do you know that stamp dealers sell more than just stamps and supplies like albums and catalogs? Although the basic stamp collector needs no more than his stamps, mint or used, there are dealers who specialized in what might be called the spice of a collection: those things that add interest and perhaps even extra value to a stamp collection.
The first items that many stamp collectors will add to their collection are first day covers. They are great in adding a touch of color to a collection, as well as further explaining the meaning of the stamp.
Related to this area are first day of issue programs, part of the actual first day ceremony. The most interesting of these are signed by the dignitaries and postal officials that attended the ceremony, although basic first day programs can also add interest to a collection. There are even those collectors who prefer their first days not in cover form, but via a stamp that has been socked on the nose (i.e., perfectly centered on the stamp) with the first day center date cancel. Such a collection is a curiosity, although one can't imagine it being worth much beyond sentimental value to its owner.
Specialty Philatelic Items
There are dealers that sell items that are the sort of by-product of stamp production -- imperfs, trial proofs, essays, etc are all part of the printing process that collectors concerned with how stamps come to market from conception to release that specialists in collecting specific stamps need to tell the whole story in their collection or exhibit.
Neophytes might want to add an item or two from this category to their collection for color and interest. But is is better to wait until the collection is well along, almost to the exhibiting stage before considering spending that big bucks that are necessary to add these sometimes rare items.
There are also philatelic items masquerading as legitimate forms of regularly issued stamps that are actually specially printed items that are produced to gain collectors' dollars. This is another reason for collectors to wait until they are advanced enough in their philatelic knowledge to make an intelligent buying decision based on facts and not dealer hype.
Dealers of Philatelic Literature
There are the basic catalogs available from dealers of philatelic literature; those without which stamp collecting would be anarchy. But after the usual Scott and Stanley Gibbons country and region catalogs, what then? More catalogs (believe it or not) but of a more specialized nature to help collectors of what in the real world would be called niche areas.
You might enjoy the collection of airmail covers and to cover that area you use the American Air Mail Catalog, published by the American Air Mail Society. But what if you want to specialize in zeppelin flights? There is no lack of reference material on these popular - and usually expensive - flights from the early part of the 20th century. The Siegel catalog is one of the definitive reference works that no collector of zeppelin flights would choose to be without. A small book, about 4 by 10 inches, it nonetheless sells for $40.
Philatelic literature does not come cheap. Getting out of the area of basic catalogs, it is often the case that the more specialized the collector wishes to become, the more expensive the reference works will be. Many have such small printings that the demand for them far exceeds the supply.
The Catalogs Back Pages (aka BOB)
Often when a collector finds a stamp that he can't identify from the basic catalog it will be the case that the stamp is a back of the book stamp. That is, a revenue, semi-postal, tax, military, local, etc. Many dealers stock these specialist items and the collector who has never ventured beyond his collecting area's postal stamps could do well to investigate some of these unusual, non-postal items. The U.S. has a great variety of revenue stamps and though the repetition of the subject of Washington can turn some off, the beauty of the design of many of these stamps makes up for it.
Many dealers advertise their specialties in the philatelic press in print and online. The stamp collector looking for something beyond basic postage stamps may also check out the ranks of dealer organizations like the American Stamp Dealers Association.