"I want you to find out who in the family murdered Harriet, and who since then has spent almost forty years trying to drive me insane." (4.195)
We wonder why Henrik never imagined that the flowers could be a sign that Harriet is alive. Probably because after all those years he didn't dare hope it could be true.
"So in effect I've spent half my life collecting information about a single day." (6.44)
Since Henrik doesn't know what happened that day, it's like he has a gap in his memory.
[…] Salander did not like aborting assignments once she had started. People always have secrets. It's just a matter of finding them out. (7.69)
Salander thrives on the secret pasts (and presents) of others, while guarding the secrets of her own past carefully.
Then, as she was on the threshold of her teens, All The Evil happened, which she did not want to think about. (12. 126)
Unfortunately, we don't get to find out what "All The Evil" is until The Girl Who Played With Fire.
[Salander:] "This is an animal sacrifice in the style of 1954 and 1960 and it doesn't seem credible that someone active fifty years ago would be putting tortured animal corpses on your doorstep today." (22.8)
Why does Martin revert to his father's animal torture by mutilating Blomkvist's friend, Tjorven the cat?
Christ Almighty. This is no historical mystery. Martin Vanger is murdering women today. And I wandered right into it… (24.10)
By exploring the past, Blomkvist comes face-to-face with very present horrors.
[Salander:] "He had that room for twenty-five years." (24.108)
That room has a hideous past. We can be sure the people who know about it will never forget it.
[Harriet:] "And when I looked up, there stood Martin." (26.116)
Harriet is telling Blomkvist about the day she kills her father. Martin saw her, and used the information to blackmail her into doing things he wants her to do. She's carried her secret for almost forty years.
Her mother's death meant the wound would never heal, since she would never now get an answer to the questions she had wanted to ask. (27.65).
This is the moment we realize that Salander herself doesn't know the secrets of her own past.