Minnesota Wildflowers (Chayka and Dziuk)
Minnesota Department of Natural Resources
These are the primary books I've used for identification, and they are all worth adding to your collection.
Peterson and McKenny, A Field Guide to Wildflowers of Northeastern and North-central North America.
Oslund and Oslund, What's Doin' the Bloomin'?
Royer and Dickinson, Weeds of the Northern U.S. and Canada.
Tekiela, Wildflowers of Minnesota