The Girl Who Played with Fire by Stieg Larsson.
"The girl with the dragon tattoo is back. Stieg Larsson's seething heroine, Lisbeth Salander, once again finds herself paired with journalist Mikael Blomkvist on the trail of a sinister criminal enterprise. Only this time, Lisbeth must return to the darkness of her own past (more specifically, an event coldly known as "All the Evil") if she is to stay one step ahead--and alive. The Girl Who Played with Fire is a break-out-in-a-cold-sweat thriller that crackles with stunning twists and dismisses any talk of a sophomore slump. Fans of Larsson's prior work will find even more to love here, and readers who do not find their hearts racing within the first five pages may want to confirm they still have a pulse. Expect healthy doses of murder, betrayal, and deceit, as well as enough espresso drinks to fuel downtown Seattle for months. -- Dave Callanan " taken from Amazon.com
This is very popular so all I'm going to say about it is that it's MUCH better than the 1st book.