The origin and "core" of the Mythos start of course with the collection 23 of Lovecraft’s greatest weird tales. In these stories, monstrous entities traverse the gulfs of time and space and humankind cowers in fright at the havoc they wreak.
And beyond this, there are plenty of Cthulhu-adjacent stories not authored by HPL that are fantastic reads as well. The Lovecraft wiki has some good examples and a nice diagram for what is colloquially deemed "canon", if someone reminds me, I can link it here if so desired.
I've been reading a collection of Innsmouth-related stories in a compilation aptly named "Shadows Over Innsmouth", very great stuff. I can't say it exactly emulates the Innsmouth feel, but I'm still loving what I'm reading so far. I do recommend it.
If you’re going to get the B&N collection get the 2011 edition (purple ribbon bookmark one). Because the 2008 version (gold bookmark) haves a ton of typos and spelling errors.
Hi, thank you for the comments. I plan to add all the anthologies (this is the first) and the books mythos-related i can find, but any suggestion is more than welcome. 👍