Anyone with a sense of the trends of the stamp collecting world knows that for some time one of the most popular modes of collecting is by topic. I have sung the praises of this and now Linn's Stamp News has come to the party big time, offering a new feature in its once a month special edition called By Topic, found in its Scott New Issues Update department. Linn's online is behind a paywall, but for any who are more than casual collectors, it is essential reading.
For some collectors the basic listings of new issues by topic such as Linn's provides are enough: they can see what is available and procure examples for their collections. But if it is more in-depth coverage of topical stamps you are looking for, you can't do better than to join the American Topical Association (ATA). Along with other benefits the group provides, you receive their publication, Topical Time, edited by Wayne Youngblood, one of the hobby's best philatelic journalists.