In order to learn about the Anglo-Saxon water mill, you should begin with "The Parts of an Anglo-Saxon Mill," by Philip Rahtz and Donald Bullough," Anglo-Saxon England Vol. 6. (1977): 15-37. (This is an annual publication and it is classified as a periodical.) You will quickly see that this discussion is highly technical and that it assumes quite a bit of knowledge about the medieval world. Don't be put off by this assumption; instead, work around it. You can learn more if you can consult other sources. In addition to encyclopedia articles about mills (in particular look for technical discussions), you should look for these:
Marc Bloch, "The Advent and Triumph of the Watermill," part of his book, Land and Work in Medieval Europe (London, 1967); a translation of Bloch's French original, first published in 1935.
Lynn White, Jr. Medieval Technology and Social Change . New York: Oxford University Press, 1962. 88-89 and the entire third chapter on sources of power.
Here are some questions that will help you learn more about mills--which, by the way, served many purposes:
1/98