The following books are filled with poisons and herbs descriptions. Most were written for specific editions of Dungeons and Dragons, but most of them can easily be adapted to any roleplaying game.
Supplement for The Guide to Herbs for RPG's
Hello Fry, it's a ... *[stops mid-sentence, throws a D20 and a D6]* pleasure to meet you. Gary Gygax voicing himself in "Anthology of Interest I" on Futurama TV show