It's quite famous nowadays, and that's mainly thanks to the movies I think. It's also becoming a Hollywood remake which I... uh, let's just say I don't really like remakes.
As of now, I've read all the three books and I've seen the movies (or, not really. I've seen the Millennium TV series that have been running on swedish television. It's like extended versions of the movies) and I'm really amazed. It's been a very long time since a book series captured me like these did.
Anyhow. The books tell the tale of the woman Lisbeth Salander and the journalist Mikael Blomqvist. I won't try to explain the plot or anything because I'm lazy and I know that one can find it easily by a short and simple google search.
What captured me in these books might have been the fact that we get to know Lisbeth part by part and unlocking the mysteries of her life as we read along. I could barely put the books away from me and when I had finished reading them it felt like something suddenly disappeared out of my life. Quite sad :/
Anyhow, I recommend these books to everyone! Really, these books (and movies) shouldn't be missed out. (Hmm, I feel like trying to draw Lisbeth... which is quite a surprise since I haven't been drawing anything seriously for more than two years now.)