Finished this story just a few minutes ago and my jaw is still hanging. The concepts Lovecraft puts forth in this story are nothing short of genius. While his writing can certainly be classified as horror, it's also important for fellow sci fans to take well deserved notice of him. This story (written over 80 years ago, mind you) has quantum concepts, alternate realities, extradimensional entities, moments of revelation that rival and even exceed some of the best sci fi i have ever read - not to mention an exceptionally lucid, comprehensible, yet appropriately alien descriptive style that only the best of authors could pull off.
In my mind there is only one word for an author capable of creating something this amazing, let alone in the 20's and 30's when he was writing.