Lovecraft not only wrote for the pulps under his own name. He also assisted other authors in revising, editing, and generally shaping their own horrific works. In some cases, only some minor touching up was needed. In others, Lovecraft took the basic idea of the story and rewrote it entirely, creating a new tale that might stand alongside his own in terms of quality. Sometimes.
Really, some of these stories are pretty obscure and should remain that way. Nonetheless, there are some wonderful pieces among these. Lovers of the alien civilizations of “At the Mountains of Madness” and “The Shadow out of Time” may find a treatment of similar ideas regarding civilization and decadence in “The Mound“. For those who like “The Call of Cthulhu“, Lovecraft returns to the lost continent theme in “Out of the Aeons“. In a more classic horror mode, there’s “The Curse of Yig” and “The Horror in the Museum“. I’d also add “The Diary of Alonzo Typer” by William Lumley, a tale of supernatural suggestion which would have been much better without the last two sentences, and “The Night Ocean“, of which S. T. Joshi says “it comes very close… to capturing the essential spirit of the weird tale”, and which is HPL’s last piece of fiction.
It took me a while to find some of those links, which is largely due to Arkham House’s legacy. When publishing Lovecraft’s works, Derleth decided that the revisions should be marketed separately from HPL’s writings and collaborations. This distinction has been reified as HPL’s work has been reprinted under other publishers, with the revisions either ignored or shuffled off to a side volume that rarely gets the same shelf space alongside the others, and in the e-texts of his fiction. Sadly, this means that less people read “The Mound” or “The Night Ocean” than dreck like “Poetry and the Gods” or weird fragments like “The Book“.
If you’re a Lovecraft fan who hasn’t dipped into the revisions, I recommend it highly. Some aren’t that good, but others rank up with the best of HPL’s regular stories. Besides, if you run into a self-proclaimed Lovecraft expert, you can test their knowledge by asking about the story in which the hero saves a kid from a mirror.
Papers Falling from an Attic Window 
I quite agree about the advisability of reading the “revisions”. The best of them tend to be a lot like HPL’s acknowledged works, but more in the pulp vein than his usual emphasis on the weird atmosphere.