A String of Pearls is a great little book by Michael Gilman, published by Turning Point Press in 1996.
It contains 108 meditations on Tai Chi Chuan , in line with the 108 movements of traditional Yang style.
Each section focuses on principles or concepts of Tai Chi, looking at the use of the Mind, Physical Structure, Energy, Pushing Hands and Strategies.
The book contains lots of little gems. I really like number 68, which emphasises connecting the Dan Tien, Yung Chuan, Bai Hui and Lao Kung points before starting your form.
I don't know if this particular book is still available but you can buy other books and materials from Michael Gilman's website, www.gilmanstudio.com