This Youtube video I made explains transmedia story telling through the example of Harry Potter.
Harry Potter is possibly one of the biggest transmedia storytelling franchises to date.
With 6 main Books, 8 blockbuster movies, spin-off books (Fantastic Beasts and Where to Find Them, Quidditch Through The Ages, Tales of Beedle The Bard, and The Cursed Child), interactive websites, merchandise, fanfiction, fan-made videos, social media fan groups and an interactive Harry Potter theme park; the Harry Potter franchise uses different forms of media to expand on and create its fictitious universe and allow the fans to interact with each other and the Harry Potter world.
This form of storytelling is advantageous for both the Harry Potter audience and the creators. It allows the fans to become more active produsers while creating more ways for the creators to make money from the franchise (which has a net worth of approximately $25 billion).