I've been thinking about composition a lot this year. Here are a few of my favorite books on the subject:
Mastering Composition by Ian Roberts
Roberts covers basic compositional information such as working with an underlying armature and thinking of big shapes. One really useful addition is a DVD that comes with the book in which Roberts takes his own paintings and changes them in a variety of ways to show how even the subtlest change can cause your eye to get stuck in one area of the painting and lose your central focus.
Creative Composition and Design by Pat Dews
This book is a nice relief from all the composition books that focus solely on painting. Dews includes a lot of her own collage work which translates to textiles very nicely. She also includes a variety of artists and their differing approaches to composition, some starting with shape and placement others with value or color.
Compositional Exercises for the Painter by Lucia A. Salemme