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Don't bother. All the best parts of any medicinal herb that actually works are extracted and enhanced as a part of modern medicine.
I think you will struggle to find any books on the topic that don't have a massively "hippy" bend to them. Most outdoor/survival books usually have some info on the properties of plants. IIRC, the SAS Survival Guide has a lot in it on plants.
As always, good advice is: don't eat it unless your sure.
My advice? Stick to the pharmacy. It's their job.