Quentin is buying a new book when Eliot shows up under the control of the mysterious power in episode 3x13 "Will You Play With Me?".
Peter and Olivia go to the book store to try and find the book with the formula in episode 2x14 "The Bishop Revival". From episode 3x06 "6955 kHz". From episode 4x16 "Nothing as It Seems".
Light finds the death notebook.