Over the years, I've read a LOT of gardening books.  Some of them I use regularly, some infrequently.  I've grouped them by type of book.  Annotations coming soon.
Books about Gardening in Minnesota and Wisconsin
Minnesota Gardener's Guide, Melinda Myers 
Birds of Minnesota and Wisconsin, by Janssen, Tessen, and Kennedy 
The Midwest Gardener's Book of Lists, by Susan McClure 
Weeds of the Northern US and Canada, by Royer adn Dickinson 
Reference Books
Botany in a Day—The Patterns Method of Plant Identification, by Thomas J Elpel 
Gardening Month by Month, by Martha Stewart 
Illustrated Guide to Gardening, by Reader's Digest 
The Book of Outdoor Gardening, by Workman Publishing 
Design Books
Garden Style, by Penelope Hobhouse 
A Garden for all Seasons, by Reader's Digest 
Books about Particular Garden Types
Country Flowers -- Wild Classics for the Contemporary Garden, by Rob Proctor 
The Cottage Garden, by Anne Scott-James 
The Decorated Garden Room, by Tessa Evelegh 
The Romantic English Garden, by Victoria's Secret 
How To Books
Step-by-Step Landscaping, Better Homes and Gardens, Meredith Publishing 
Step-by-Step Successful Gardening, Better Homes and Gardens, Meredith Publishing 
The Big Book of Gardening Skills by the editors of Garden Way Publishing 
The Complete Backyard Planner -- Hundreds of Ideas to Help Homeowners Make the Most of Their Homes, by L. Donald Meyers 
Just for Fun
Miss Rumphius, by Barbara Cooney 
Flower Confidential -- The Good, the Bad, and the Beautiful, by Amy Stewart 
Wicked Plants -- The Weed that Killed Lincoln's Mother & Other Botanical Atrocities, by Amy Stewart
Currently Not Classified
Bloom and Blossom -- The Reader's Guide to Gardening, edited by Mary Swander 
Great Gardens from Everyday Plants, by Anne Halpin 
Little Herb Gardens -- Simple Secrets for Glorious Gardens - Indoors and Out, by Brennan and Luebermann 
Rodale's Successful Organic Gardening -- Perennials, by Rodale Press