An interesting travelogue but it is not horror. We only know it is meant to be terrifying because the narrator tells us so. Multiple times. I found the writing to be at times too... much. As if, if it hadn't been Lovecraft, an editor would have stricken many passages as being self indulgent. And to that point, how many times can you fit the phrase "mountains of madness" into a novella?
That said, I did enjoy it. It hints enough to make you curious about the rest of this mythos if you aren't already familiar. I spent a good deal of time after reading looking up illustrations - it just stays with you.