Did they ever explain why he started writing about Storm again instead of Nikki Heat?
This is all purely speculation on my part, but Castle says in ‘Flowers for Your Grave’ that writing Derrick wasn’t fun anymore. It had become work, and he had lost interest in the character. And I think he had, because killing someone is pretty final. And I don’t think he had any intentions of bringing him to life.
If you look at the time of publication, the first Derrick Storm stuff (the three e-books) came after he met his dad in Paris. And in those books there is a kidnapping (that goes really wrong), a smart, savvy redhead. Not hard to see that Castle wrote those as an effort to purge the effects of Alexis’ kidnapping and because his dad is a real spy. There is a similar scene in Wild Storm where Derrick waterboards a man, but does so with an interesting level of humility which I think is an effort to understand the terror Kate clearly went through with Vulcan Simmons.
The first full Derrick Storm novel, Storm Front, is actually dedicated to his dad. The one that just came out, Wild Storm, is dedicated to Kate and if you read the book there’s plenty of commentary on their relationship from the old married couple who spend time apart because of a kidnapping (hint, hint), to the violatile and passionate relationship between Storm and Clara Strike (I’d wager she’s gained a flavor or two of Kate Beckett, their sniping at each other is very indicative of that), to the cute flirty banter between Storm and a graduate student named Katie Comley (KC, I snorted a little).
I think, in short, meeting his dad sparked something new in Castle with regards to Derrick. He was interested in him again, and maybe Kate encouraged him to do something beyond shadow them at the precinct and write about their life together (didn’t work, sorry KB).