That sounds like William Hope Hodgson. He was in an artillery company rather than an ambulance driver. He wrote The Night Land and The House on the Borderland, two of my favourite novels.
William Hope Hodgson was a prolific British author. H.P. Lovecraft lists Hodgson works to be among his greatest influences. His works are also known to have inspired Clark Ashton Smith and Henry S. Whitehead. Among all his most influential works you can find:
The House on the Borderland (1908)
While en route to a week of fishing in the quaint Irish village of Kraighten, Messrs Tonnison and Berreggnog stumble upon a hidden gem - a remote house perched precariously close to a yawning abyss. This secluded dwelling, once owned by a reclusive soul, holds a dark secret. Uncovering a weathered diary, the gentlemen delve into the chilling experiences of the previous occupant. The diary chronicles a descent into madness fueled by terrifying supernatural encounters and haunting visions of otherworldly dimensions, hinting at an evil far more insidious than anything Messrs Tonnison and Berreggnog could have ever imagined.
Another author worth mentioning is Algernon Blackwood and his short story The Willows was praised by H.P. Lovecraft as the finest supernatural tale in English literature
Two friends are midway on a canoe trip down the Danube River. Throughout the story Blackwood personifies the surrounding environment—river, sun, wind—and imbues them with a powerful and ultimately threatening character. Most ominous are the masses of dense, desultory, menacing willows, which "moved of their own will as though alive, and they touched, by some incalculable method, my own keen sense of the horrible."
The Night Land by William Hope Hodgson have one of the craziest world building one can image
The Sun has gone out: the Earth is lit only by the glow of residual vulcanism. The last few millions of the human race are gathered together in a gigantic metal pyramid, the Last Redoubt, under siege from unknown forces and Powers outside in the dark. These are held back by a Circle of energy, known as the "air clog," powered from the Earth's internal energy. For millennia, vast living shapes - the Watchers - have waited in the darkness near the pyramid: it is thought they are waiting for the inevitable time when the Circle's power finally weakens and dies. Other living things have been seen in the darkness beyond, some of unknown origins, and others that may once have been human.
https://www.youtube.com/watch?v=OQjS9nGrM_U
https://www.youtube.com/watch?v=Jy_RiohWtXE&list=PLyXmh8JiqrctE7CF56JFTS12O4mco-b2d
H.P. Lovecraft's Cthulhu Mythos is a shared universe far larger and more terrifying than that of humanity, where ancient, malevolent beings known as the Great Old Ones slumber in the depths of space or time. After Lovecraft's death, the Mythos has been expanded and developed by many authors, including August Derleth, Clark Ashton Smith, and Robert E. Howard. These and many other authors have helped to flesh out the Mythos into a rich and complex Dark Universe.
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