This is really the second half of the second book. There is no real separation other than the fact that it is too long for a single book.
Larsson split the two books in a good place. There was a nice cliff hanger and it made sense. But this third book picks up immediately after the conclusion of the second.
Again, this book is fairly focused on Lisbeth (my preferred main character) but because of the result of the second book her actions are restricted. In many ways, this book is about Lisbeth allowing people into her life and understanding her need to depend on others.