Exhalation stories - Ted Chiang. I'm into short stories while I physically and mentally prepare for Captive War #2.

Anyways, the first story about a gateway that can take you forward/back 20 years, but you can't change anything. Going forward merely tells you where you'll end up, going backwards merely increases your understanding of how you got where you are. Great story, fully locked in, the rest haven't caught my imagination quite like the first though.