I've been reading about William Morris's textiles, and so came across the work of his daughter, May Morris, who ran the embroidery workshop for her father's firm. She was an extremely accomplished needlewoman and designer, as well as a teacher. Her book Decorative Needlework can be downloaded from the link. It is a little gem, including a history of embroidery, a selection of stitch techniques, and an excellent introduction to the elements of design and the specifics of designing for textiles. May's style is very readable, if somewhat quaint. She is quite passionate and opinionated about what constitutes good design, but is also trusting of the reader's common sense as to the best approach of working a piece.
P.S. John Hopper has a lovely article about May Morris on The Textile Blog.