Not having read this yet, I can only assume that it wasn't Mr. Estleman himself who gave the definition of "downriver" as being "anyplace not in Detroit itself."
A lifelong resident of a downriver community, I feel the need to clarify. The term "Downriver" refers to the cities that are, for the most part, literally "down the Detroit River, south from Detroit." In other words, it includes the cities south of Detroit that actually are bounded by the Detroit River on the east (River Rouge, Ecorse, Wyandotte, Riverview, Trenton, Gibraltar) and a few cities extending to the west (including Allen Park, Lincoln Park, Southgate, Taylor, Woodhaven, Brownstown).
If Mr. Estleman is responsible for the incorrect definition, it will be my FIRST disappointment with him since I started the Amos Walker series. In any case, I look forward to finally getting a copy of this from my library and delving in.