I am often asked to recommend reference material or field guides. I thought I'd pass on two of my favorite field guides for wildflowers.
Roadside Wildflowers of the Southern Great Plains by Craig C. Freeman and Eileen K. Schofield; published by the University Press of Kansas. This is an excellent source for, as the name implies, the Great Plains. I have found that it is fairly well confined to the Great Plains, therefore not very useful east of Kansas. However, the format is easy to follow and the pictures are excellent. So many of the earlier field guides have good information and plant descriptions but the photos are of poor quality and make it hard to distinguish subtle characteristics. Not so with this book.

The second book I recommend, and probably my favorite of the two, is Tallgrass Prairie Wildflowers; text by Doug Ladd and Photos by Frank Oberle. This is a Falcon Field Guide, published in cooperation with the Nature Conservancy. This book, too, has an easy to follow format. It provides enough information to make reliable ID but doesn't get bogged down in a lot of technical jargon, and what there is, is listed in a glossary for your reference. This book covers the most common prairie plants from Kansas east to Ohio. The pictures are superior. If you are familiar with Oberle's work you know what I mean. Frank Oberle is the only photographer I have met who can catch the emotion of the prairie in a photograph. In my opinion, this book is well worth it's cost just for the photos, let alone it's usefulness as a guide. A unique feature of this guide is a grass ID section in the back. Very few field guides offer that. You will also find a directory of tallgrass prairies in 13 states.