The 5th Edition Herbalism Guide for RPGs".
I had an older copy I was using, but this newer edition looks so much nicer. Basically the same information, just laid out better using a more familiar "old school" font. Much easier on the eyes.
I especially love some of the information compiled in the appendices. Truly great stuff to be had there.
I've found a couple versions of this guide. 1st there is this online copy, view-able here. The second versions, which is a complete PDF, has download links available in this RPG.net forum post. I'm not including direct download links here because a) that's not how I roll (pun intended) and b) if the authors change distribution channels they would most likely update the link.