Having just had an exceptionally busy week, I've been trying to recall the details of a relevant short story whose name escapes me. I have the feeling it was a fairly old, 'classical' horror story - probably not by Edgar Allan Poe, but in his kind of style - and also quite famous and well-known.
The central character is a lawyer, or lawyer's clerk, who is in the process of combing through some massive volumes in order to piece together enough evidence to implicate a particular person in a crime. I think he's working to a deadline; as the days drag on, he becomes more and more devoted to his search, and gets less and less sleep. At the end of the story, he finishes collecting the final pieces of evidence he needs, and promptly collapses from overexertion. In an epilogue, he wakes up in hospital days later and hears that he was successful in producing enough evidence for conviction.
There is also a supernatural, and somehow parallel, element to the story. I can't remember the details of the demonic creature involved - I think it had a name, but like many of the best horror villains, perhaps its true nature remained insidious and unclear even to the very end - but somehow the revelation of this monster was intimately connected to the clerk's search for evidence. It was something like: the less sleep he got, the stronger the creature became; or the closer he came to finding the evidence he needed, the closer the creature came to materialising. In any case, the climax of the story was the climax for both parts: as he completed his search for evidence, the demon also reached some critical stage. It's possible that the demon didn't actually exist and was just a product of his fevered imagination, since I'm pretty sure he made a full recovery in the epilogue and was no longer plagued by it.
Again, I think this is a pretty well-known story, and I expect someone else here will have heard of it. There may be some elements I'm not remembering precisely right, but hopefully this will be enough for someone to identify the story.
Any clues as to what it might have been called?