Originally asked and answered over on Quora, how much should we consider non-Lovecraft created works as legitimate canon?
You shouldn’t so much as ignore the Cthulhu Mythos outside of what was originally written by H P Lovecraft as recognize its use for what it is.
Keep in mind that the “Cthulhu Mythos” as labeled was an invention of August Derleth. August Derleth reinterpreted and frankly misrepresented some key features of the universe as presented in Lovecraft’s original works, and even went on to write “posthumous collaborations” in which he took some of Lovecraft’s common book notes and created entirely new stories – none of which were actually read or authorized by Lovecraft. Several essays in ST Joshi’s Lovecraft and a World in Transition as well as works of many scholars in Dissecting Cthulhu: Essays on the Cthulhu Mythos go into extensive detail about this.
Many subsequent authors and works were created based on August Derleth’s interpretations or instead, cherry picked Lovecraft to suit the stories they wanted to write.
During Lovecraft’s lifetime, he encouraged his friends to make use of his inventions. That doesn’t mean he accepted their interpretations though as canon extensions – merely that he was okay with it, and may also have praised them. The best example of acknowledgement was that of the stories associated with Robert Bloch.
Bloch write a story called The Shambler from the Stars (in which a Lovecraft stand in is killed), and Lovecraft delightfully wrote a sequel called The Haunter in the Dark (in which a Robert Bloch stand in is killed). Bloch then followed this with yet another sequel to Lovecraft’s story, called The Shadow from the Steeple.
So what should you consider “canon”?
Anything that Lovecraft created himself or borrowed and put into his own stories should be considered canon. I would extend this to include Bloch’s The Shambler from the Stars since Lovecraft liked it enough to write a sequel.
Nothing else should be considered canon, but anything that Lovecraft acknowledged should at least be given some credit as ‘authorized’.
But don’t write off the works of other authors. There are some fantastic Lovecraftian stories from many authors, including Bloch, Robert E Howard, and many others – and also from authors that had no relation at all to Lovecraft or that first generation of Lovecraftian writers. Some are very, very tenuously related to Lovecraft.