I read
At The Mountains Of Madness for the first time last month, and jotted down some thoughts on it and wider Lovecraftian themes in my blog:
Long Read: Gott ist tot, but Yog-Sothoth Lives! The perfect historical oddity of H.P Lovecraft
Suggestions for Further Reading:
I Am Providence: The Life and Times of H. P. Lovecraft by S. T. Joshi
Some Notes on a Nonentity by Sam Gafford & Jason C. Eckhardt
Ave Atque Vale: Reminiscences of H. P. Lovecraft ed. S. T. Joshi & David E. Schultz
Lovecraft Remembered ed. Peter H. Cannon