I wouldn't say it's a history source, though one can see a good deal of early 20th century xenophobia and racism (and occasionally sexism) in his works, The Shadow over Innsmouth and At the Mountains of Madness are good for this, particularly if one's reading the annotated version.

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And yes, I will say that if one looks at the works past his disgust for people who weren't WASPs, there was still interesting stuff in there that would be a major influence on later horror, as well as societal fears about technology and what scientific discoveries meant for views of humanity. Just not one to read uncritically.