There are three books I use when keying out minnows. Do a google search on Amazon for the following, listed in order of cheapest to most expensive...
Iowa Fish and Fishing - has no fish key unless you get the antique version from the 50's. I recommend this book as a supplement to one of the other two. It gives fish ranges in the state that are "fairly" accurate.
Fishes of Missouri by Phlieger - excellent fish key and shows the ranges as well. Is mostly a Missouri book but Iowa and Missouri fish populations are pretty similar. You can buy this book for about $25 shipped from the Missouri Department of Conservations web page. I use this book A LOT.
Fishes of Wisconsin - very very nice book, but expensive, look to spend over $125 for it. I don't own my own, yet.
I would get the Iowa book and the Missouri book, that will allow you to identify 95% of the minnows in our state... if you can figure out the technical jargon.