Lovecrafts work are short stories, this one only being about 100 pages. The subject matter is almost exclusively people losing their sanity over the inability to comprehend the horrors they’re seeing but follows the genre of ‘what you can’t see is scarier than what you can’. I enjoyed mountains of madness but the writing structure being as dated as it is can make it somewhat difficult to read so I always recommend the audiobook version. If you’re looking for another good short horror story try “I have no mouth and I must scream”. It’s only about a 40 min audiobook.