Early Buddhism
Below you'll find resources related to early Buddhism. By early Buddhism, I mean the teachings of the Buddha as recorded in the Sutta Pitaka and parallel texts – prior to the doctrinal developments of later traditions, including Theravada. That said, even within the Sutta Pitaka, some texts are likely later additions.

Sutta Pitaka - The Numerical Discourses of the Buddha, The Book of The Fours, sutta 180 (AN 4.180):
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"Here, monks, a monk might say: ‘In the presence of the Blessed One [Buddha] I heard this; in his presence I learned this: 'This is the Dhamma; this is the discipline; this is the Teacher's teaching!' That monk's statement should neither be approved nor rejected. Without approving or rejecting it, you should thoroughly learn those words and phrases and then check for them in the discourses [sutta] and seek them in the discipline [vinaya]. If, when you check for them in the discourses and seek them in the discipline, (you find that) they are not included among the discourses and are not to be seen in the discipline, you should draw the conclusion: ‘Surely, this is not the word of the Blessed One, the Arahant, the Perfectly Enlightened One. It has been badly learned by this monk.’ Thus you should discard it."​​​​
Sutta Pitaka
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Digha Nikaya (Maurice Walshe)​
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Majjhima Nikaya (Bhikkhu Nanamoli and Bhikkhu Bodhi)
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Samyutta Nikaya (Bhikkhu Bodhi)
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Anguttara Nikaya (Bhikkhu Bodhi)
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Khuddaka Nikaya
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Dhammapada (Acharya Buddharakkhita)
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Udana (Thanissaro Bhikkhu)
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Itivuttaka (Thanissaro Bhikkhu)
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Suttanipata (Bhikkhu Bodhi)
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Scholarly Works
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What the Buddha Thought (Richard Gombrich)​
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How Buddhism Began (Richard Gombrich)
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In the Buddha's Words (Bhikkhu Bodhi)
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Works by Bhikkhu Analayo
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Playing With Fire (Joanna Jurewicz)