Recommended Reading
Mindfulness with Breathing: A Manual for Serious Beginners
Ajahn Budhhadhasa Bhikkhu
Lays out the practice of anapasati meditation, literally mindfulness of the in and out breath.
"When you know for yourselves, ‘These things are wholesome; these things are blameless; these things are praised by the wise; these things, if undertaken and practiced, lead to welfare and happiness,’ then you should engage in them.”
~ The Buddha
AN 3:65
Lays out the practice of anapasati meditation, literally mindfulness of the in and out breath.