The Color Encyclopedia of Ornamental Grasses

Drake, Rick

A Gardener's Encyclopedia of Wildflowers

Burrell, C. Colston

A Well Tended Perennial Garden

Sabato, Tracy D.

The Native Plant Primer

Ottesen, Carole

The New England Wildflower Society Guide to Growing & Propagating
  Wildflowers of the US & Canada

Cullina, William

Native Trees, Shrubs & Vines: A Guide to using, growing &
  propagating North American Plants

Cullina, William

Plants of the Chesapeake Bay: a guide to wildflowers, grasses,
  aquatic vegetation, trees, shrubs & other flora

Musselman, John & Knopper, David

Peterson First Guide to Wildflowers of Northeastern & North
  Central America

Peterson, Roger Tory

A-Z Encyclopedia of Garden Plants

Brickell, Christopher

Armitage's Garden Perennials: A Color Encyclopedia

Armitage

Rodales's Illustrated Encyclopedia of Perennials: 10th Anniversary
  Ed.

Rodale