"The Devotion of Suspect X" is a mystery novel by Keigo Higashino that follows the story of a brilliant mathematician named Isagaki and his neighbor, Yasuko Hanaoka, who becomes embroiled in a murder case. After Yasuko kills her abusive ex-husband in self-defense, she seeks help from Isagaki, who devises an intricate plan to cover up the crime. As the investigation unfolds, Detective Kusanagi and his friend, Yasuko's daughter, become suspicious of Yasuko's involvement. The novel explores themes of love, sacrifice, and the lengths one will go to protect those they care about, all while maintaining a gripping narrative filled with clever twists.