Online psychedelic drug archive Erowid has scanned in a copy of a classic guide to hallucinogenic plants of the world and how they are used by native peoples.
The Golden Guide to Hallucinogenic Plants is by pioneering ethnobotanist, Richard E. Schultes.
Ethnobotany is the study of how people make use of plants, and hallucinogenic plants are obviously of keen interest owing to their important place in ritual and religion throughout the world.
The book is sadly out of print and and second hand copies are now collectors items. However, the full version has been scanned in full colour, so you can read it online or download it as one single zip file.