Vapors

The sort of thing I consider a "SI done right" -- the protagonist doesn't start with mythical "chosen one" powers, doesn't have game-breaking knowledge-from-outside, and has no encyclopediac knowledge of the source universe.

Fantastic read all the way up to a couple chapters before the end, at which point there's a big cliched twist when the story could have just ended in a supremely satisfying way. The sequel is much shorter, deals with that twist, and has a satisfying ending of its own, but the story probably would have hung together better if that twist hadn't occured. All-in-all a great read, though, even with that issue.

Sequel: Clarity.