I just loved Helen Parrott's Mark-making in Textile Art. This a kind of book you're looking for when you set out to make a journey and learn something new. It gives you ideas, offers advices, shares experiences in regards to both the practical and the creative aspects of the creative work. I loved her emphasis on exercising your mark-making on paper. It's so much faster and you can get a good idea, what the resulting stitched piece might look like.
These are the books I found most useful. Do you have any other good suggestions? Please leave comment and share with us!