Where the ideas actually start
Not with a plot. With a rule someone in the story is willing to break.
The question readers ask most often is where a book begins. The honest answer is that it almost never begins with a plot. Plot arrives late, and it arrives as a consequence.
It begins with a rule. Not a law — laws are the least interesting rules people follow. The rules that generate stories are the unwritten ones: what a family does not discuss, what a profession will not admit, what a room expects of the people standing in it. These rules are usually invisible, because everyone obeys them. They become visible at the exact moment someone decides not to.
So the first note in a file is rarely a character or an event. It is a sentence like: in this world, no one asks where the money came from. Or: in this building, an accusation costs more than the thing being accused. Once that rule exists, the story more or less starts asking its own questions. Who is most constrained by it? Who benefits from everyone else obeying it? What would it take to make someone break it in public?
The murder, when it arrives, is the answer to those questions rather than the premise. It is the cost of the rule failing. That is why the investigation can matter beyond the puzzle — because solving it means naming the rule, and naming the rule implicates everyone who kept it.
The Glass Veil began this way. So did The Helios Directive, at a different scale. Both started as a single line about what a particular world refuses to say out loud.