There certainly is not the one best book on this.
Modern historical books may even be less plagued than medieval books on the topic, but there will be a considerable difficulty awaiting you:
the setting described is "a village" but most of the primary texts we can study are really high level science texts in comparison. Oral lines of traditions are the ones most frequently cut.
We know of ancient medicine from antiquity (VN), presenting us a huge corpus of medical writing, which was the basis or did form most of the 'proper' medical knowledge in medieval Europe. There is clear continuity in this knowledge of the learned strata of society, building up and "standing on the shoulders of giants". But we also know that a substantial amount of practical knowledge was actively disregarded, discarded or which was simply lost. Mainly those of mid-wives, witches, and those farmer's almanach level practices of which we only have hints at in the established texts.
And especially the physical methods were widely known and in some cases practised for thousands of years before writing came along (eg Trepanning).
That would men to set the cut-off date for such a novel during the renaissance, but include everything we know of from before. Such traditional medical knowledge should be the main focus, while the academic medicine might have been indeed trickling down into the villages, but access and availability might have to be considered.
In terms of principal systems to understand, the foremost would be the corpus hippocraticum as completed by Galen which gave us the system of humoral pathology (4-bodily juices). Those need to be in balance and harmony. This is a departure from magical thinking in medicine, but not a complete one: Medicus curat, natura sanat, deus salvat. (The medic acts, nature heals and God saves).
The deus part might read as a Christian addition, which it is, but in pre-Christian Europe this were simply other gods and deities to appeal to. Read that as shamanistic practices, praying and sacrifices still play a big role throughout.
In terms of diagnostics you'd have to emphasise acute and meticulous observations, like uroscopy, pulse diagnosis (PD), bloodletting (BL), coproscopy (CS), and the same for sweat and sputum.
Traditional healers had more at their disposal than just herbalism, but the academic basis for this would be found in works like De Materia Medica. Keep in mind that this is Mediterranean in principle and the higher up North the setting is chosen the less applicable some plants are, as they do not grow there but have to be traded, again a potential problem in "villages".
Perhaps a better intersection of academic doctors and traditional healers might be found in monasteries, where clerical monks recorded their experience on a level somewhat 'below' (CM)
Steve Parker: "Kill or Cure? An illustrated History fo Medicine", DK: New York, 2013.
David Wootton: "Bad Medicine. Doctors Doing Harm Since Hippocrates", Oxford University Press: Oxford, New York, 2006.
- More focus on medieval healing and medicine:
Daniel McCann & Claire McKechnie-Mason (Eds): "Fear in the Medical and Literary Imagination, Medieval to Modern –– Dreadful Passions", PalgraveMacMillan: London, 2018.
Thomas Glick & Steven J. Livesey & Faith Wallis: "Medieval Science, Technology and Medicine. An Encyclopedia", Routledge: London, New York, 2003.
Luke Demaitre: "Medieval Medicine. The Art of Healing, from Head to Toe", Praeger: Santa Barbara, Denver, 2013.
Anne Van Arsdall & Timothy Graham: "Herbs and Healers from the Ancient Mediterranean through the Medieval West. Essays in Honor of John M. Riddle", Ashgate: Farnham, Burlington, 2012.
Faye Marie Getz: "Healing and Society in Medieval England. A Middle English Translation of the Pharmaceutical Writings of Gilbertus Anglicus", University of Wisconsin Press: Madison, 1991.
Timothy D. Walker: "Doctors, Folk Medicine And The Inquisition: The Repression Of Magical Healing In Portugal During The Enlightenment (Medieval and Early Modern Iberian World)", Brill: Leiden, Boston, 2005.
Marcia A. Kupfer: "The Art of Healing: Painting for the Sick and the Sinner in a Medieval Town", Pennsylvania State University Press: University Park, 2003.
Sheila Campbell & Bert Hall & David Klausner (Eds.): "Health, Disease and Healing in Medieval Culture", Palgrave Macmillan: , 1992.
Nancy G. Siraisi: "Medieval & Early Renaissance Medicine. An Introduction to . Knowledge and Practice", University of Chicago Press: Chicago, London, 1990.
(VN): Vivian Nutton: "Ancient Medicine", Oxford University Press, Oxford New York, 22013.
(PD): Evan Bedford: The Ancient Art of Feeling the Pulse. In: Brit. Heart J. Band 13, 1951, p423–437.
Emmet Field Horine: An Epitome of Ancient Pulse Lore. In: Bulletin of the History of Medicine. Band 10, 1941, p209–249.
(BL): Peter Brain: "Galen on Bloodletting: A Study of the Origins, Development and Validity of his Opinions, with a Translation of the Three Works"
K. Codell Carter: "The Decline of Therapeutic Bloodletting and the Collapse of Traditional Medicine", Transaction Publishers, New Brunswick, London: 2012.
(CS): Franz Knoedler: "De egestionibus. Texte und Untersuchungen zur spätmittelalterlichen Koproskopie." Pattenesen: Wellm, 1979.
(CM): Hildegard von Bingen & Bruce W. Hozeski & Gretel Ehrlich: "Hildegard's Healing Plants: From Her Medieval Classic Physica", Beacon Press: , 2001.
As it was classified as "fantasy novel", you really have a huge range of options or inspiration to draw from. If you also look at Byzantine, Arabic, Indic and Chinese medicine. A traveller or trader might bring in some knowledge from there. Parallele devlopment. Besides herbalism, you might also use minerals and metals, like arsenic, quicksilver (mercury) special salts, 'stange' animal products etc. Just like traditional Chinese medicine, ayurveda and so on.
Even when going for strict "historical accuracy" in 'good practice medicine' you might be free to include things that would seem like outright quackery. Medics of all times have had strikes of genius for treatments and at the same time did things we would have to classify as pure butchery. Just think of how until recently we had tonsillectomies done routinely or that grandmas still tell you to keep your throat warm so as to not 'catch a cold', and that we indeed have no cure for the common cold.
A sometimes dreadful, or hilarious, read for that:
J.C. McKeown: "A Cabinet of Ancient Medical Curiosities. Strange Tales and Surprising Facts from the Healing Arts of Greece and Rome", Oxford University Press: Oxford, New York, 2017.
In my opinion, the present state of medical knowledge represents the complete discovery of the art, for it is able to give precise instructions about the nature of diseases and explain the essential aspects of their treatment (Hippocrates Places in Man 46).
Many aspects of treatment in antiquity were very stressful and unsophisti- cated (Pliny Natural History 26.16).
It is not easy to explain surgical procedures in writing (Hippocrates Joints 33).
You should clearly understand that bandaging a broken jaw efficiently does little good, and bandaging it inexpertly does great harm (Hippocrates Joints 32).
In seeing the desired cut-off point for knowledge, a great study of writings would be Paracelsus. Deviating from humorism and criticising it loudly, covering a broad range of theory and treatments.
Andrew Weeks: "Paracelsus (Theophrastus Bombastus von Hohenheim, 1493–1541)
Essential Theoretical Writings", Brill: Leiden, Boston, 2008.