The characters' gullible acceptance of highly dubious material leads to an examination of their deeper mental depths and, lo and behold, they find monsters. When I first read this book some twenty five years ago, even a callow youth such as I wondered if this was the diary of a delusionary, a tale told by a lunatic. The fantastic events of the story aren't supported by the peculiar dream logic one finds in Lovecraft or Van Vogt, but are made up of only a simple minded acceptance of flawed and ambiguous evidence. This story totters for two reasons. First, mainstream writers approach science fiction at their peril.
Genre is a bit like playing the violin, easy to do badly and difficult to do well.
Secondly is the weakness shared by much of the post-Lovecraft Cthulu fiction, that the visionary imagination, the inherent madness in the original, is lacking. Much as Philip K. Dick could approach paranoia with a knowing perspective, Lovecraft was clearly a seriously damaged individual, comfortable with a peculiar and vivid madness. In his damage was the genius of his stories. The sane are not meant to tread the paths of madness that Lovecraft knew. Nonetheless, the book is highly effective in some ways.