Once again, Prof. Henry Jenkins of USC's Annenberg School has posted a syllabus for his Transmedia Storytelling and Entertainment class (Fall 2013) online.
While I highly recommend studying the entire syllabus (you will gain insights into texts, topics and how a leader in the field approaches teaching transmedia storytelling), I will excerpt just one introductory paragraph here to illustrate how Prof. Jenkins succinctly introduces the concept of a transmedia story:
"A transmedia story represents the integration of entertainment experiences across a range of media platforms. A story like Heroes or Lost might spread from television into comics, the web, alternate reality or video games, toys, and other commodities, etc., picking up new audiences as it goes and allowing the most dedicated fans to drill deeper. The fans, in turn, may translate their interests in the franchise into concordances and Wikipedia entries, fan fiction, vids, fan films, cosplay, game mods, and a range of other participatory practices that further extend the story world in new directions. Both the commercial and grassroots expansion of narrative universes contribute to a new mode of storytelling, one which is based on an encyclopedic expanse of information which gets put together differently by each individual, as well as processed collectively by social networks and online knowledge communities."