From what I've read HPL exclusively published (or at least submitted) his works to periodicals and magazines. Were any of his works primarily published, primarily as a book?
It was first published after his death in the Weird Tales periodical, but has since been republished as a novel.
Here's two of the covers it's had:
¹I can't seem to find much information on The Inevitable Conflict, but it appears to also have been first published in another periodical. One site claims it may not have even been written by Lovecraft.