Mutant City Blues feels more like a collection of suggestions for a setting than a traditional book including setting and rules ; It's all good if you are willing to play a US based super mutant investigation, otherwise you'll have to do the legwork by yourself.
The GUMSHOE system, at least the way it is presented in this book, lacks definition for my taste ; a player friendly set of pools, with clear instructions for the GM that failure does not exist and has little consequences on the investigation outcome.
The order in which informations are provided makes little sense, with references, acronyms and rules that are explained long after being used into gameplay exemples.
All in all, you'll find pretty good ideas in mutant city blues setting suggestions, but don't expect a solid foundation for a campaign, or a game ready to go... Therefore, the 20 USD pricetag seems a bit much.