Armitage’s Garden Perennials A Color Encyclopedia by Allan M. Armitage, Timber Press

Dirr’s Hardy Trees and Shrubs An Illustrated Encyclopedia by Michael A. Dirr, Timber Press

Gardening in the Lower Midwest A Practical Guide for the New Zones 5 and 6
by Diane Heilenman, Indiana University Press

Gardening with Native Plants of the South by Sally Wasowski with Andy Wasowski, Taylor Publishing Company

Trees for American Gardens The Definitive Guide to Identification and Cultivation
by Donald Wyman, MacMillan Publishing Company

The Well-Tended Perennial Garden Planting and Pruning Techniques by Tracy DiSabato-Aust, Timber Press

Reader’s Digest The gardener’s Book of Color by Andrew Lawson, Reader’s Digest

The Complete Shade Gardener by George Schenk, Houghton Mifflin Company

The Native Plant Primer by Carole Ottesen, Harmony Books

And, if you have gone completely over the edge:

Manual of Woody Landscape Plants Their Identification, Ornamental Characteristics, Culture, Propagation and Uses- 5th Edition by Michael A. Dirr, Stipes Publishing LLC

Herbaceous Perennial Plants A Treatise on their Indentification, Culture and Garden Attributes by Allan M. Armitage, Stipes Publishing, LLC

Manual of Herbaceous Ornamental Plants 4th Edition by Steven M. Still, Stipes Publishing LLC

Of additional interest:

A Natural History of Trees of Eastern and Central North America by Donald Culross Peattie, Houghton Mifflin Company (Read the passage on the American Beech, and weep)

Gardener’s Latin by Bill Neal, Algonquin Books of Chapel Hill

Home Ground A Gardener’s Miscellany by Allen Lacy, Houghton Mifflin Company

The Secret Garden by Frances Hodgson Burnett

Botany for Gardeners an Indroduction and Guide by Brian Capon, Timber Press

...and many many more.