I'm a fan of finding terror in places you don't expect it.
Spoilers for The Time Machine
The Time Machine, by H.G. Wells, is a beloved sci-fi adventure story following the accounts of an inventor trapped in the far-flung future, and his efforts to decipher the nature of the two eventual evolutions of humanity, divided into the tranquil teddy bear like Eloi, simpleminded humanoids living carefree on the surface, and the Morlocks, a subterranian monster race stalking an intricate web of steel tunnels below the surface...
What is essentially a delightful time travel story builds up to a climax wherein the Time Traveler managed to daringly escape back to his Time Machine. Then proceeds to undermine the entire story by having said Time Traveler, driven by sheer curiosity, ride further and further into the future in progressive stages. Slowly, as he continues forward, the Earth dies. Humanity devolves further and further from the relatively recognizable Eloi and Morlocks into an unfathomable shell, indistinct from anything the reader or the Time Traveler can understand or comprehend.
And that's when The Time Machine, by H.G. Wells, shows its hand, and reveals that its been a cosmic horror story all this time.
There is no Azathoth here, however, only the impenetrable, unstoppable hand of time, as it wipes away the past. Gradually, at first, but throughout the millenia, slowly all things are washed away, lost, absolutely forever. Vanished to Time.
I love this book and have been fascinated by it ever since I finished reading it on a train in Salt Lake City, carrying the folded book in my armpit as I walked back from the ride. It's never left my mind. What was at first the science fiction adventure I'd been looking for my whole life -- truly contemplative, deeply immersive storytelling in a strange land, utilizing the all too unexplored "far future" as opposed to the simpler near future 23XX style tales of men in space ships on alien planets, to thread a gripping yarn -- pulled back all the layers, exposing me to the desolate, shivering core of the story that had been lurking there all this time. The fate of humanity, truly gone, even in the time of the Eloi and Morlocks, lost to the ages, didn't dawn on me until Wells literally drove the point deeper and deeper, as the Time Traveler pushed himself further into the future.
There's an "official sequel" called The Time Ships by a personal favorite author of mine, Stephen Baxter, which was written and released with the authorization of the Wells Estate 100 years after the publication of the first book. I've owned it for a while, never really got deep into it, because the premise of a sequel didn't sit right with me. The prose seemed authentic enough, but the running premise -- The Time Traveller returns to the future to resuce one of the Eloi he got killed by accident -- seemed forced.
I've started reading it and I'm absolutely blown away. As a sequel, this is fascinating. Piling on the existential dread by forcing not only the reader but the Time Traveler himself to come to terms with things truly being lost forever, and the mental gymnastics that makes your brain undergo.
I won't spoil any more of it here, I genuinely recommend these books even if I may poke fun at myself for constantly recommending them. Didn't give the "sequel" enough credit until now, it's shaping up to be something interesting.