This is the last of the Millennium trilogy by Stieg Larsson. The first part, The Girl With The Dragon Tattoo, can be read separately. I think the 2nd, The Girl who Played With Fire, can be read separately, too; but it's recommended to read the first book first. However, we cannot read The Girl who kicked the Hornets' Nest without at least reading the 2nd book.
In the first, the heroine Lisbeth Salander helps journalist Mikael Blomvkist to solve a 40 year-old case of a missing girl. In the 2nd, Salander's past is explored. It explains why she is declared incompetent and has to be under guardianship, even though a few people who know her say that she is a genius. The 2nd book ends with Salander confronts her father, the mysterious Zala. They almost kill each other.
In this book, Salander is under arrest for attempting to kill her father and several other charges. Her friends, including Blomvkist, who believe her story about a conspiracy to lock her in mental hospital forever; work to save her and uncover the conspiracy. The enemy, known as 'The Section' seems to hide behind the government, but as Blomvkist will reveal the truth in his next book and magazine issue about how the government has violated Salander's rights for years, the government must act and work together with Blomvkist to remove the corrupted officers.
Too bad the writer was dead after finishing the 3rd book. If there was the 4th book, what would have it been about? Salander's lost sister?