Materia Medica = 'medical material'
body of knowledge regarding the therapeutic properties of medicinal substances
The following is a list of resources I consider indispensable to my own Materia Medica and personal library of holistic healing. Please remember:
1. Understanding the body is as important as understanding the herbs you intend to treat it with. Herbalism requires a working knowledge of both.
2. Any herb (or substance) can be a poison in the wrong dose or when used inappropriately. Always do your homework: check not just your books but the world wide web for the latest & most up to date dosage & contraindication information. 
3. If you would like to eventually prescribe herbs to anyone other than yourself, find out what type of herbal practitioners your state allows and explore the offerings of herbal apprenticeships and academic programs available near you.

INTRODUCTION TO HERBALISM

Holistic Herbal
by David Hoffmann
Simple materia medica for common western herbs

The Herbal Handbook
by David Hoffmann
Broad introduction to herbal actions

Encyclopedia of Medicinal Herbs
Andrew Chevallier
An extensive materia medica with concise monographs & color photos


Herbal Healing for Women
by Rosemary Gladstar
Holistic approaches to women's health

The Web That Has No Weaver
by Ted Kaptchuk
Introduction to traditional Chinese concepts of diagnosis & treatment

The Yoga of Herbs
by David Frawley and Vasant Lad
Introduction to Ayurveda concepts & herbs

AROMATHERAPY & BOTANICAL SKIN CARE 

Encyclopedia of Essential Oils
by Julia Lawless
Detailed profiles of essential oil properties & characteristics

Aromatherapy with Chinese Medicine
by Dennis Willmont
Advanced aromatherapy combinations & application using traditional Chinese treatment concepts

Artisan Perfumery
by Alec Lawless
Introduction to perfumery & oil blending

Liquid Sunshine
by Jan Kusmirek
Carrier & vegetable oil profiles with composition info

Beauty by Nature
by Brigitte Mars
Easy recipes for making natural skincare at home

ADVANCED WESTERN HERBALISM

Medical Herbalism
by David Hoffman
Herbal constituents & body systems with a concise Western materia medica

The Energetics of Western Herbs Volumes 1 & 2
by Peter Holmes
Western Materia Medica written with an advanced Eastern perspective on herbs {Chinese diagnostics, energetics & treatment principles}

ADVANCED EASTERN HERBALISM

Materia Medica
by Dan Bensky
Comprehensive Materia Medica of Chinese herbs

Formulas & Strategies
by Dan Bensky 
Advanced Chinese formula compositions & applications

Dui Yao

by Philippe Sionneau
Excellent resource on Chinese herbal pairs


NUTRITIONAL THERAPY

Healing with Whole Foods
by Paul Pitchford
A nutrition & dietary guide written with an Eastern perspective but in layman's terms

BOTANY

Botany in a Day
by Thomas J. Elpel
Approachable guide to basic botany & plant family characteristics

HAWAII PLANTS

Plants in Hawaiian Medicine
by Beatrice H. Krauss
Common medicinal Hawaiian plants & their uses

La'au Hawai'i
by Isabella Aiona Abbott
Traditional uses of Hawaiian plants

Medicine at Your Feet
by David Bruce Leonard
Extensive guide to local plants with medicinal value & cross-cultural comparisons of use

Plants & Flowers of Hawaii
S.H. Sohmer & R. Gustafson
Color photos for identifying island flora

Hawaiian Forest Plants
by Mark Merlin
Color photos with distribution & uses of Hawaii's forest plants

ENTHEOGENS

Plants of the Gods
by Albert Hofmann and Richard Evans Schultes
Psychoactive plant profiles with social & contextual history

The Encyclopedia of Psychoactive Plants
by Christian Rätsch
Extensive guide of all known psychoactive species including botany, history & distribution