When Castle learns P.J. Moffet has been murdered, he mentions that Moffet only published one book in his lifetime. This could be a reference to author Harper Lee, who, for the majority of her life, also only published one book, To Kill a Mockingbird (1962). A few months before her death, she published "Go Set a Watchman"; while it was initially promoted as a sequel, it has since been accepted as a first draft of "Mockingbird".
When Martha shows up at the precinct to talk to Beckett about the separation, Beckett firmly declares that she and Castle aren't getting divorced. During their investigation in "A Chill Goes Through Her Veins (2009)", when Beckett still considered Castle an annoying nuisance, they roleplay being a married couple, in the course of which she comments "If we're married, I want a divorce".