The Devotion of Suspect X" is a Japanese mystery novel by Keigo Higashino, known for its intricate plot and surprising ending. It follows a divorced single mother, Yasuko, and her mathematically gifted neighbor, Ishigami, who become entangled in a murder cover-up after Yasuko's ex-husband unexpectedly reappears and dies. Detective Kusanagi and his friend, physicist Dr. Manabu Yukawa, investigate, and their efforts to uncover the truth lead to a complex game of deception.