This is the major U.S. stamp organization. Information on how to join is on the site.
Membership gets benefits, such as a subscription to their monthly magazine.
This is a great web site sponsored by the The Collectors Club of Chicago. The leading U.S. periodical. Some articles are on-line, and you can try the magazine free of charge. Postal Stationery of Denmark 1871-1879 Types and errors of the first Postal Cards of Denmark. Lots of pictures of unused and used Postal Stationery.