What can I say about this mystery except that I felt deliciously guilty keeping it out of other readers' hands? Or ears, since I listened to the audiobook. (Thankfully the narrator used British accents, even though the characters were Swedish.)
Journalist Mikael Blomkvst is hired to re-investigate the disappearance of a wealthy industrialist's niece, under the guise of writing his biography. (This happens in the aftermath of Blomkvst being convicted of libel when he prints a story that seems true but whose sources wish to remain anonymous. Subplot A.)
The case of Harriet Vanger is especially cold, because she disappeared 40 years ago, and it's also a vexing one. A traffic accident on the bridge of the island she and 30 or 40 other people lived on cut the island off from the mainland the day that she vanished.
Blomkvst eventually teams up with Lisbeth Salander, a hacker who works for a security firm as a researcher, but who has a strange past that has somehow led the state to declare her mentally incompetent and in need of a state-appointed guardian (subplot B).
I'm looking forward to reading the next two in this trilogy given how fast-paced, original and yet plausible The Girl With the Dragon Tattoo was. If you'd like to read a similar book that's actually in the library right now, you could try one of Arnaldur Indridadson's mysteries.