This won't take long. Think of a trilogy, published in 2005, 2006 and 2007, after the author's death, which became a worldwide best-seller, combining kinky sex, computer hacking, and political intrigue at the highest level. Not since Harry Potter in the very first of these entries have I had a series which so completely dominated the books set in a particular country; and in contrast to Harry Potter, these three are very much set in the gritty reality of contemporary Sweden (with a certain allowance for dramatic licence). They are, of course:
The Girl with the Dragon Tattoo / Män som hatar kvinnor, by Stieg Larsson
The Girl Who Played with Fire / Flickan som lekte med elden, by Stieg Larsson
The Girl Who Kicked the Hornets' Nest / Luftslottet som sprängdes, by Stieg Larsson
To turn to something much more wholesome, the fourth-placed book is also the first of a trilogy (I had thought there were more, but no, there were only three) for children, first published in 1945, about an eccentric and very rich nine-year-old who lives alone with her monkey and horse and is befriended by two local children. She is, of course:
Pippi Longstocking / Pippi Långstrump, by Astrid Lindgren
Nothing else really comes close.