Culbreth's Materia Medica (1927)
Abridged version of the original manual, downloadable in Adobe Acrobat (pdf) format. 443 pages, including over 300 illustrations of medicinal plants.

An Illustrated Herbal
An illustrated herbal based mostly on Culpeper's Herbal. It contains around 500 entries of medicinal plants and their properties. Glossary of terms, preparation methods, bolean search engine.

Henriette's Herbal: Classic Herbal Texts
Large collection in html format.