Judy Balchin's "Art Nouveau Designs" uses simple, but elegantly rendered, line drawings to give the beginning user a real sense of the flow and curve of the Art Nouveau style. The patterns are uncluttered enough for easy tracing and transfer, with plenty of room for the more advanced artist to add their own embellishments. Black and white illustrations are ideal for photocopying, and can be scaled using a modern copy machine that does it automatically.
My only caveats are that the book is heavily skewed towards floral designs (about 15 pages out of the 32 page total), and that the human figures are rendered in a flatter style (see simple) than you'll find in similar motifs elsewhere, especially in reproductions of period references. That being said, this book is an excellent resource for those new to the exuberant Art Nouveau style, and a good way to get a feel for the structure underlying the works of Mucha, Mackintosh, Lalique, and many others.