"World Sourdoughs From Antiquity" by Dr. Ed Wood.
A wonderful work on sourdoughs. Some people don't like his recipes for breads, as they aren't purist enough for them. Whether or not you like his recipes, he sets the standards on how to handle cultures. A new version is supposed to be out soon. Highly recommended. isbn.nu can point you to a 1989 paper back, and a a 1996 paperback.