Publishers and booksellers are in a rush to find more Nordic noir to follow Stieg Larsson’s Millennium trilogy, known for the indelible characters of Ms. Salander and the investigative journalist Mikael Blomkvist. The books have become a publishing phenomenon, selling 6 million copies in the United States and 35 million copies worldwide — nearly four times the population of Sweden. The third and final book in the series, “The Girl Who Kicked the Hornet’s Nest,” was published last month in the United States by Alfred A. Knopf and instantly became the must-read book of the summer.