Here are a few books to help in a study of period medicinal knowledge and practice. In no way are we recommending the use of any of these books or their contents for medical practice (unless you have water-elf disease).
Siraisi, Nancy G. Medieval & early renaissance medicine: an introduction to knowledge and practice. Chicago, University of Chicago Press, 1990 250pp. illustrated. [An excellent overview of medical "knowledge" and practice. The notes and bibliography are a good springboard for further study.]
Rawcliffe, Carole. Medicine & society in later medieval England. London, Sandpiper, 1990. 241pp. illustrated. [This book provides more of the social context for medicine than Siraisi's book. It looks at medicine for the non-physician. It also contains extensive bibliography, and wonderful pictures.]
Arikha, Noga. Passions and tempers: a history of the humors. NY, HarperPerennial, 2007. 376pp. illustrated. [To understand medieval medicine, one must understand the science behind it. Humoral theory was not only part of medicine, but also of nutrition, psychology, and even martial arts.]
Hildegard von Bingen (1098-1179). Physica: The complete English translation of her classic work on health and healing. Rochester VT, Healing Arts, 1998. 250pp., illustrated. [A medical treatise by the twelfth-century abbess, composer, mystic, and political critic. Hildegard claims to be "uneducated". I don't believe it.]
Hildegard of Bingen. Hildegard’s healing plants: from her medieval classic Physica. Translated by Bruce W. Hozeski. Boston, Beacon Press, 2001. 192pp. [An herbal extracted from Physica.]
Tobyn, Graeme. Culpeper’s medicine: a practice of western holistic medicine. Rockport MA, Element, 1997. 278pp. illustrated. [Culpeper is post-SCA period, but some of his ideas were firmly rooted in medieval thought. Culpeper shows that one needn't go to the far East to find holistic health theory and practice.]
Culpeper, Nicholas (1616-1654). Culpeper’s complete herbal: a book of natural remedies for ancient ills. Ware, Wadsworth, 1995. 603pp.
Arber, Agnes. Herbals: their origin and evolution. Glastonbury, Lost Library, 1912. 253pp. illus. [This is not about medicine per se, but about the books that transmitted medical knowledge. It was revised in 1938, but this edition is more widely available.]
Adamson, Melitta Weiss. Food in medieval times. (Food through history series) Westport CT, Greenwood Press, 2004. 256pp. illustrated. [Medieval "nutrition" was informed by humoral theory. This book illustrates the connection.]
Root-Bernstein, Robert. Honey, mud, maggots, and other medical marvels: the science behind folk remedies and old wives’ tales. Boston, Houghton Mifflin, 1997. 279pp. [Medieval medicine may look like superstitious nonsense to those who believe in artificial drugs. This book shows the modern science behind early therapy.]
Freeman, Margaret B. Herbs for a medieval household: for cooking, healing, and divers uses. NY, Metropolitan Museum of Art, 1943. 48pp. illus. [A very short herbal, illustrated with period woodcuts. It's not all that informative, but it's a delight.]
Grieve, Mrs. M. A modern herbal: the medicinal, culinary, cosmetic and economic properties, cultivation and folklore of herbs… NY, Barnes & Noble, 1996/1931. 912 pp. illus. [On the other hand, this is a doorstop of an herbal. Some of her information comes from medieval sources; much comes from Culpeper and later sources. It's available online at http://www.botanical.com/botanical/mgmh/mgmh.html]
Rodrigues, Louis J. Anglo-Saxon verse charms, maxims & heroic legends. Pinner, Middlesex, Anglo-Saxon Books, 1993. 166 pp. [This is primarily a book of poetry from the 10th and 11th centuries, but it also contains charms against various diseases, conditions, and social problems. This is the book to have if you're troubled by water-elf disease.]