I don't usually rec stories, because I'm not very good at putting into words why I like a story (as opposed to why I dislike a story, which is easy). So, I'm just going to use most of what I commented to rec this one..
For 96K, this reads very quickly/flows very well. I love the complexity - the multiple types of magic and science (without going into paragraphs of pure explanation) and the layers and complexities to their relationship. The plot kept me guessing (which, honestly, rarely happens), and the tension between Severus and Harry was excellent, changing just enough as the situation altered to keep it from becoming stale/feeling drawn out.

Really, others said it better, especially perverse_idyll. If you haven't read (or started to read) this one yet, please do (heed the warnings, though, if you're the type to read warnings at all)

Title: Anachronism
Author: Starcrossed
Team: Team Cauldron!
Genre(s): Alive and Kicking
Prompt(s): Seeking Knowledge and Doppelgänger
Rating/Warnings/Kinks: NC17, (the warnings are visible after the link in the story header, just highlight to see them where directed)
Word Count: ~96k total.
Summary: Upon waking up, Severus finds himself not recovering from Nagini's bite, but healing after a long-term mysterious illness. Not only must he uncover the mystery of the cause of his affliction, but also deal with his lover, who is the very last person he wanted to ever see again: Harry Potter.

Read it Here
http://asylums.insanejournal.com/snarry_games/252197.html

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In other news, I'm off in the morning on a work trip to denver and salt lake city (a bit of a whirlwind tour, not like I'll get to actually see/do anything) and then to visit Bironic  (YAY!!). I don't know yet if I'll be taking my computer / have internet access, so I may not be commenting until next tuesday.