When I was a small child child, I found this book on my mother's bookshelf and fell in love. Small wonder I'm a giant nerd now.
Herbs for the Mediaeval Household for Cooking, Healing and Divers Uses Freeman, Margaret B. (1943)
It's out of print, but the publisher (The Metropolitan Museum) has put up an OCR'd PDF for free download, with the original art and layout intact.
The text is based on 17th-18th century herbals, which in turn are based on herbals going back to the ancient greeks. It's layed out in a readable version of medieval blackletter and formatting (no paragraphs, just inline paragraph marks) and some archaic punctuation. It is illustrated with 15th century engravings. There's also some modern commentary on the herbs.
This book is screaming to be the herbalist PCs reference manual.
I was delighted to find it again online. It's a shame I can't share the physical object with you, but thanks to the generosity of the publisher I can at least share the beautiful content.
EDIT: another good one from the same site is
Sweet Herbs and Sundry Flowers: Medieval Gardens and the Gardens of The Cloisters.