If you are interested in just skimming the surface of the subject, I'd suggest:
Herbs For The Medieval Household For Cooking, Healing And Divers Uses. Margaret B. Freeman
New York, The Metropolitan Museum Of Art, various printings

Or, you can check out the books by Rosetta Clarkson and Eleanor Sinclair Rohde. I'd suggest Clarkson's Green Enchantment and Magic Gardens both of which were also published under their subtitles. Very readable, if dated.