It just recently dawned on me that they haven't mentioned the Nikki Heat movies anymore on the show. They had that one episode with the potential actress portraying Nikki Heat shadowing Kate but that was it... unless I'm forgetting.
I kinda wish they'd bring it up more. I liked the idea of the books being made into movies in the Castle-verse and the whole idea of how it complicated Kate's job a little, not to mention seeing her squirm at being dragged into the limelight. Maybe once their relationship is leaked, she'll have to deal with the limelight again. Oh man, the media would have a field day with the fact that Castle and his inspiration behind Nikki Heat are an item!
They've also stopped mentioning the Nikki Heat books except for maybe on one episode a season around the time when a Nikki Heat novel is actually released here in the real world. I thought it was hilarious when the first book came out and Kate was hiding in a bathroom stall, trying to find the sex scene in the book and Castle pops his head over the stall to tell her what page.
I hope they return to mentioning the whole Nikki Heat thing more!