however Mahayana traditions have added much to the Buddha's original teachings that were not specifically there. It does not mean that their interpretations or all of their 'additions' are wrong per se.
Theravada traditions are those that hold most closely to the Buddha's original teachings. You will find these in the original Pali Canon. I recommend Bhikkhu Bodhi's translations.
Without some background, they can be pretty heavy reading. So as you are an atheist, I highly recommend you start with Stephen Bachelor's Buddhism without Belief & Confessions of a Buddhist Atheist.
Most works here in the US on Insight Meditation and Mindfulness will also be devoid of the later metaphysics and added supernaturalism. Though I must stress that even Mahayana traditions see those as human psychological projections and the strong need for many to explore the more metaphysical aspects of life. They are NOT necessary to 'enlightenment'.
I would also recommend Chogyam Trungpa's books especially Cutting through Spiritual Materialism which dispels many westerners' ideas of Buddhism and guru's in particular. Finally if you have an interest in Buddhist psychology, explore the writings of Nyigma Institute out of CA. Tarthan Tulku's writings are exceptional especially the psychology and philosophy ones.
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