“Theravada (pronounced — more or less — “terra-VAH-dah”), the “Doctrine of the Elders,” is the school of Buddhism that draws its scriptural inspiration from the Tipitaka, or Pali canon, which scholars generally agree contains the earliest surviving record of the Buddha’s teachings.”
— accesstoinsight.org

Introductory Material:
The Buddha’s Teachings As It is by Ven. Bhikkhu Bodhi
Mindfulness in Plain English [free ebook] by Ven. Gunaratana

Intermediate Material:
In The Buddha’s Words by Ven. Bhikkhu Bodhi
The Four Foundations of Mindfulness in Plain English by Ven. Gunaratana

Advanced Material:
Practicing the Jhanas by Stephen Snyder and Tina Rasmussen
Satipatthana: The Direct Path to Realization by Ven. Analayo

Pali Nikaya Translations:
Digha, Majjhima, Samyutta, Anguttara [free anthology]