It's always interesting to hunt down authors' inspirations. One of the largest influences underlying Howard Phillips Lovecraft's work - at least as far as his mythos works are concerned - seems to me to have been William Hope Hodgson's The Night Land; a book you have to work to find.
Well, unless you go to Project Gutenberg and just click.
HPL is important because of his influence on modern horror, fantasy and speculative fiction. He's remembered for his imagination and world-building, and has become a popular target for charges of racism and misogeny (to which historians can only point mutely to the 20s in the United States and ask if accusers have any knowledge of how deeply racist and sexist the US was at that time).
Hodgson is important because he influenced Lovecraft heavily. The Night Land doesn't just hint at the Cthulhu Mythos of HPL's works, it does a very good job of trotting out the final Mythos story wherein the Great Old Ones are back and looking for lunch.
I'm not going to offer a scholarly discussion of the book, other than to warn that it, like Lovecraft's fiction, is written in a dense and difficult manner (Hodgson wanted to force a strange pseudo-archaism in the book's style, and it requires some effort to read); rather, I suggest you fight your way through it and learn a bit about the inspirations for the works that went on to inspire authors and filmmakers whose works you read, or watch, today.
Besides, there's an interesting story in there, set in a world so post-apocalyptic it make Mad Max look like Anne of Green Gables.