Recursion

The story revolves around a memory device that can bring people back in time to a memory that they then can change -- but it changes everyone else's memories, and they can then remember those timelines as well. Mindbending fun.

Interesting concepts about memory and time and how screwing around with both/either can have catastrophic consequences. I didn't think the world building or the characters were very well-developed, but it did make for an entertaining story.