In the middle we have Lovecraft himself. At this point I’m pretty sure I’ve read all his stories including the ones that were revisions of other people’s stories. Except for the occasional excerpt I haven’t read any of his letters. He wrote far more letters (perhaps as many as 100 thousand, often many pages long) than he wrote stories and a good chunk of his correspondence has been collected and is in print.
For me, a big part of the appeal of the Cthulhu Mythos, is its collaborative nature. Anybody can write a story and add to it. That started with Lovecraft. He added his creations to the stories he revised. He referred to other’s creations in his stories. His friends referred to his creations in theirs. The Mythos is inconsistent and contradictory. Fans argue over which stories should be considered canon. While many Mythos stories are under copyright the Mythos itself is in public domain.