Have just finished another Hugo-nominated novella, Palimpsest by Charles Stross. This is a very intricately woven time travel story, and very well told, probably the strongest of the lot I've read so far. The time entanglements in the story make it clear what a pain time travel would be if it were possible--times become more like places, infinitely accessible and mutable. There is a sense of time outside of time--the sense of bodily decay, I'm guessing. Some have speculated that that is in fact what time is, the sense of entropy. There's no rules in physics that say time has to run one way.
There is also a novel named Palimpsest that is nominated for a Hugo. It's a fun word to say--palimpsest, palimpsest, palimpsest. I expect to see it in a Zippy cartoon sometime.