There have been so many books over the years that gave me insights that I needed to understand and learn from. Some are: The Tree of Yoga, The Noble Eightfold Path of Buddhism, The Dhammapada, Raja Yoga, The Buddha's Ancient Path, In This Very Life, Patanjali Yoga Sutras, and Speaking of Yoga: A Practical Guide to Better Living. A monk in Thailand some years ago told me that I needed to study "The Big Book". When I asked him what this was, he told me that it was myself. So I would say that studying oneself through meditation and breathing and within the context and light of the Buddhist teachings is and has been the most important and useful book for me and perhaps others also.
For me (after many years of reading books on buddhism on and off) I picked up "The Three Pillars of Zen" by Philip Kapleau roshi. I read it and when I put it down i felt that it was time to just do this practice myself and not just read about it. It has been an ongoing source of inspiration ever since.
Other very inspiring books for me have been: "Zen Mind; Beginner's Mind" by Suzuki, "The Bodhisattva Precepts" by Reb Anderson, "The Practice of Perfection" by Robert Aitken roshi, "Sex, Money, War, Karma" by David Loy, "Being Peace" and "Teachings on Love" and "Commentaries on the Heart Sutra" by Thich Nhat Hahn, "The Teachings of Bodhidharma" by Red Pine, "Moon in a Dewdrop" by Dogen, "Wild Ivy" by Hakuin.... Well, it feels like this list is endless :)
For me, it was Pema Chodron's 'The Wisdom of No Escape'. I had a series of traumatic experiences in a very short time. Like her, I was, and still am for the most part, experiencing a feeling of being completly groundless. The title alone penetrated me to the core, and I realized quickly that my trauma was self-inflicted, and I had been trying my entire life to escape from myself. This started me on an entirely new life-path, and I am only now taking my first steps on that path. Namaste, Pema Chodron. I highly recommend it to everyone.