You would be hard pressed to find someone who hasn't at least heard of this series. Originally written by Stieg Larsson in Swedish, these books have taken the world by storm. In fact, the stories are so popular, a Hollywood movie is already in the works (a Swedish version was released in 2009). Sadly, Stieg died before any of the books were published, so he never experienced the raging success of his novels.
The stories in each book revolve mainly around Lisbeth Salander (THE Girl) and Mikael Blomkvist, with many subsidiary characters playing their designated parts. I've heard a lot of people say they are hard to get into initially - for me, it took awhile to figure out where the stories were headed (particularly in the first book), but I had no trouble becoming engaged from the beginning. For anyone not familiar with Sweden, the names of places in the novel (actually names in general) can be a bit of an obstacle. Overall a decent series, but be forwarned - there is some serious graphic content in these novels!
Rating: ♥♥♥½
Side-note: Apparently Larsson was in the process of writing a fourth book in the series when he died, but there's no word on whether someone will be contracted to finish the story.