Below are the best books we could find on Heartmath.
Heartmath is a biofeedback technique based on the guiding principle that your heart is a source of intelligence. By listening to it—in addition to your logical brain—you gain helpful guidance throughout life. Essentially, the heart is seen as a second brain. The theory focuses on heart rate and rhythm as an indicator of emotional state. A secondary principle is that your heart will actually sync to the rhythm and rate of others’ emotions as well, serving as a kind of mirror to those around you. The principle and the term itself stem from the Heartmath Institute, founded in 1991, which provides resources for synching your physical, emotional, and mental states to achieve a more harmonious existence.