Caught in the Act!
Castle gets Beckett a copy of his new novel HEAT WAVE. But she's nonchalantly says she hasn't read it yet. This drives Castle crazy. Little does he know that Beckett is secretly reading her favorite mystery writer's latest bit of literature after hours. Her clandestine indulgence involves the comfort of a glass of wine and a bubble bath.
The next day at the precinct Castle makes an offhand reference to the lead character's sex scene and what page it was on. As soon as Castle leaves the room, Beckett flees to the restroom and thumbs through HEAT WAVE to that very scene.
Unfortunately for her, Castle peeks over the closed stall doors and catches his biggest fan in the act (of reading, that is). Of course, Castle is as happy as a kid at Christmas. He knew all along she had been reading his new book.