Aspiring writers are often told to write what they know, and what crime writers know is crime writing. Yes, others write crime: former cops, private investigators, prosecutors. But transforming real-world knowledge into fiction is not an easy or common deed. Unless it is executed very skillfully, a focus on writing in a narrative can be tremendously dull. For a rare example of how to do it well, see the eight seasons of “Castle,” a TV show about a crime writer who hangs out with cops to gather material. The show works because we never reckon with Castle’s writing life: Somehow, he’s a bestseller who doesn’t ever sit in front of his computer playing solitaire or checking his Amazon ranking.