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I've read the top four books. I didn't know that there was so much more than Tolkien wrote.
The LOTR feels like a fully fleshed out universe, yet surprisingly little is actually mentioned in detail, it seems only in passages from the mythology backstory books that the detail is revealed, but the detail was ever-present in the trilogy because it made it all believable.
If you can draw a better network, feel free to copy this.