THE DREAM-QUEST OF UNKNOWN KADATH by H.P. Lovecraft
The edition being used is that printed in the Victor Gollancz “At The Mountains of Madness and Other Novels” (1966) hardback book – pages 290 to 385. I have found what I consider to be many startling things in this novella.
“…but being old in the land of dream he counted on many useful memories and devices to aid him.”