Zeppelin II
Star Wars Empire Strikes Back
Harry Potter and the Chamber of Secrets
Perhaps the best ingredient to all of these was the gap between the first one and the second one. After the first one was heard or seen or read, the wondering began.
How is it going to start? What is it going to feel like? Who is involved? How similar is it going be to the first one? What happens next?
Now we can consume all the volumes all at once. We don’t wait nine months after listening to Zeppelin I before listening to II just because that’s what people had to do in 1969. But the fact that they did have to wait for it is part of what made it stick (and opening with Whole Lotta Love didn’t hurt).