You definitely have to buy the books to easily access the islands – and it’s one island per book. The activity books include a DVD, activity sheets, and stories. The DVD contains cartoons – they’re not bad, either – including the same voice acting from the islands.
The islands are incredibly long – I went through Family Island and collected pictures for (hopefully) all the characters in the island. It turned out to be 90 characters – 90! A regular island on Poptropica has about 30!
And yes, I plan to document the game on the Poptropica Wiki. It may take some time. 😛 From what I learned, the background art in ODI is incredible – they don’t let the 2D boundary get in their way. They use 3D trickery to make the world seem much more immersive.
Oh, one more thing: Tropical Island held a surprise for me. While playing through the island, it showed a family tree, and what appeared to be Mordred from Astro-Knights Island! I’m not completely sure that this is canon, but it definitely showed Mordred’s parents! Sadly, I didn’t get a screenshot, but hopefully I will later.
What does this mean? Do these universes crossover? Tell me in the comments!