iancanton wrote:why does the
book him! region need to be a killer neutral rather than, say, have decay of 99 troops per round?
ian.

The problem, as I see it, is that once you take "Book him!", your opponents can never allow you to have a notepad set until someone else takes "Book him!" from you. This creates a permanent change in the structure of the game where whoever holds "Book him!" cannot be allowed to hold a set. It also makes taking "Book him!" too easy for anyone who wants to take it after the first person. This doesn't make the map unplayable, but I don't like that dynamic existing permanently until the end of the game.
It also doesn't make much sense logically, that you could continue to hold someone even if you no longer have the proof that you used to arrest them. I think it would be possible to make it match the original Clue game, where a false accusation is a losing condition, and I think transformations would allow me to set the neutral troops the the original value, but that would discourage the use of the win condition until one player has an overwhelming advantage. So I thought the best option from a playability viewpoint would be if it was a killer neutral. That way, someone can try to get the win condition, but if they fail, then the game goes back to the way it was.