Some Observations on Philatelic Literature
The most useful item on most stamp collectors' bookshelves is a catalog. If you are in the US, you are probably destined to have a Scott's. Unless you collect all the current stamps or price by Scott's, you don't have to have the latest edition. If you collect another country, you should probably have a catalog which holds the same position as Scott's in that country. For example, British Commonwealth collectors should have a Stanley Gibbons; collectors of Germany will appreciate a Michel.
Although Scott and the other catalogs generally have good introductory materials, a good general reference is always useful. I find I use Cabeen's Standard Handbook of Stamp Collecting frequently and even occasionally browse it to learn more. It provides the second level knowledge about a lot of subjects after you've decided you want to know more than the definition of a term. (BTW, Sutton's The Stamp Collector's Encyclopaedia is good for definitions, explanations of items, and finding stamps from printing on the stamps.)
Far and away my favorite US reference work is Brookman's three volume The United States Stamps of the 19th Century. They are the starting point for any of those issues. Johl's books do well for the first third of the 20th century. I haven't found great enough need for similar books of the latter part of the Century.
Thorp-Bartels Catalogue of United States Stamped Envelopes is a wonderful introduction to the world of US postal stationery. I have had my copy of the century edition for over 35 years (even have an extra).
Herman Herst's books (e.g. Nassau Street) are just fun to read and you often can find some philatelic gem there. Sloane is fun to read as well although he doesn't have the stories to tell that make Herst so famous.
One book I still use and enjoy is Colorado Postal History by Bauer, Ozment and Willard. In many respects this was a pioneering work. I also know the authors. Jack Willard was kind enough to give me a proof copy for marking up with notes.
I don't want to ignore the periodicals. An example of how they are useful is illustrated by the specialist articles on the dies of the Columbian stamped envelopes. The first work was done by Ellis and reported in an article in one of the Stamp Specialist volumes in the 1940s. The next article was part of a serialized article on Commemorative Stamped Envelopes in the American Philatelist in the 1970s. In the early 1990s there was an expansion which added a subfamily published in the American Philatelist. These comprise an extended amount of information on these dies which makes collecting them easier and more interesting. (There's also the article which I want to write on the keys to using the meridians below the heads for rapid, simple and reliable identification of the die types. I will do this when I get a Round Tuit! :-) )