Watch this great intro to the ambigram, a typographical treat

Ambigrams are words or symbols styled so they still have meaning when viewed from another perspective. This video is a great overview of some of the best artists and books on the craft.

Probably the most popular creator of ambigrams is John Langdon, and his seminal book is called Wordplay, where both his name and the title are ambigrams:

Ambigram Art (YouTube / DONG)