Which of Tolkien's Stories Are Part of the the Middle-Earth “Canon?”

The question of what counts as "canon" in Tolkien's Middle-earth writings is thorny and convoluted, and devolves pretty quickly into guesswork, but let's see if we can narrow things down a bit.
Hello Future Me's video on the unreliable narrator is here: https://www.youtube.com/watch?v=3P-brBxPFJk
For my video on the Hobbit retcon, click here: https://www.youtube.com/watch?v=eIbXfJ72j4c
And for the translation conceit, click here: https://www.youtube.com/watch?v=qhyQ1pzDDLQ

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