Self-help books can be a good complement to therapy. While they're not a substitute for the therapeutic process, they may shine a light on a particular issue you are dealing with in therapy.
Here are a few books we have found helpful over the years:
How to Get the Love You Want, Harville Hendrix, PhD
The Seven Principles for Making Marriage Work: A Practical Guide, John Gottman, PhD and Nan Silver
A New Earth: Awakening to Your Life's Purpose, Eckhart Tolle
Passionate Marriage: Keeping Love and Intimacy Alive in Committed Relationships, David Schnarch, PhD