I use Paul Prudhomme's "Louisiana Kitchen" book. It has good instructions and more importantly images showing the different levels of color. A gumbo needs a very dark roux, it looks like used motoroil. It is a fine line between getting it right and burning it. I like to go slow to avoid pushing it too far.
I lived in Louisiana for several years, so I have seen it done and more importantly know how it's supposed to taste like!
Of all Cajun/Creole dishes I still make, Gumbo is the one I make most often. If I have any tough small game critters that need a good boil to be tender, Gumbo is the go-to dish. You can put anything in a Gumbo. That's the spirit!
G