Celtic Design: Knotwork: The Secret Method of the Scribes by Aidan Meehan
Covers the basic how-to of Celtic design, including step patterns, key patterns, spirals, uncial script calligraphy, and decorative letters. For more detailed information, see Meehan's other titles in the Celtic Design series. Includes plenty (258) of very well done black and white illustrations demonstrating the techniques described in the text.