What are the essential sources for the Cthulhu Mythos?

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I fell in love with Lovecraft's stories awhile back and wanted to learn more about the Cthulhu Mythos. However, I know the Mythos has grown beyond what Lovecraft originally wrote, so attempts to study it usually ends with me propped up in the middle somewhere. Where is the best place to start to get the whole story, if there is one?