Several authors have tried to write books on the constuction of Insular Art--Celtic Knotwork. Each has achieved various degrees of success, and no one author can be recommended as THE author to buy. If it is at all possible, try to get the works of George Bain, Iain Bain, Courtney Davis, and Aidan Meehan through the Interlibrary Loan system. Try each author's method of construction, see which works for you, before buying. And, actually, my personal favorite set of knotwork instructions comes from a one page article, "Interlacing Without Erasing" by Melinda Sherbring, published in a corporate publication, The Known World Handbook.