From amazon.com:

The sixth Lisbeth Salander story in the Millennium Series - the crime-fiction phenomenon that has sold more than 90 million copies worldwide.

Mikael Blomkvist is trying to reach Lisbeth Salander - the fierce, unstoppable girl with the dragon tattoo. He needs her help unraveling the identity of a man who died with Blomkvist's phone number in his pocket - a man who does not exist in any official records and whose garbled last words hinted at knowledge that would be dangerous to important people. But Lisbeth has disappeared.

She's sold her apartment in Stockholm. She's gone dark. She's told no one where she is. And no one is aware that at long last she's got her primal enemy, her twin sister, Camilla, squarely in her sights.

In the end, it will be Blomkvist - in a moment of unimaginable self-sacrifice - who will make it possible for Lisbeth to face the most important battle of her life, and, finally, to put her past to rest.