What Investigative rules and advice is in Mutant City Blues but not elsewhere in GUMSHOE games?
I'm not an expert on Mutant City Blues, but from skimming through the book:
The Quade Diagram is a big influence. (This is the diagram that shows how the mutant powers are linked to each other). For example, you can infer that someone has High Energy Dispersal because you've found evidence of Lightning and Gravity Control, and HED is between those two in the diagram.
There is a lot of stuff about procedure. Mutant City Blues investigators are police and must follow procedures. For example, they can't just go into someone's dreams and find out information: there are procedures forbidding that.
Interviewing suspects is a big thing. They're more roleplay guidelines than rules; I haven't had the chance to try them yet.
Forensics are a big thing too. Lots of information about that in the rulebook.
The structure of investigations is different from other Gumshoe games: Backstory, The Crime, The Investigation, The Twist, The Culprit. Thus, it's crime-based.