Truth is, no one knows why they were smash successes. There's far too many variables involved to boil it down to some sort of success road map.
In the end though, they had a story that appealed across markets (YA & Adult), they contained elements readers found appealing (romance, action, mystery, etc.), and readers flat out enjoyed them.
I read a lot of those books, simply because I wanted to know why they were such smash hits. One thing I found in common is they all were easy reading. That's part of the reason people scoff at their readers and writers, talking about the "horrible writing". Is the prose on the level of Nabokov or Tolstoy? Certainly not, but if someone thinks writing so that reading is easy and unaffected is simple or banal, they haven't a clue. It's damn hard work to write so the reading is easy. I believe a large portion of readers, those that wish to be entertained primarily, truly appreciate easy reading. That factor can serve to heighten enjoyment because it's not work, while at the same time deepening immersion.
In the end though, they had a story that appealed across markets (YA & Adult), they contained elements readers found appealing (romance, action, mystery, etc.), and readers flat out enjoyed them.
I read a lot of those books, simply because I wanted to know why they were such smash hits. One thing I found in common is they all were easy reading. That's part of the reason people scoff at their readers and writers, talking about the "horrible writing". Is the prose on the level of Nabokov or Tolstoy? Certainly not, but if someone thinks writing so that reading is easy and unaffected is simple or banal, they haven't a clue. It's damn hard work to write so the reading is easy. I believe a large portion of readers, those that wish to be entertained primarily, truly appreciate easy reading. That factor can serve to heighten enjoyment because it's not work, while at the same time deepening immersion.