Larry Niven. Rainbow Mars. Tor, 2000 (originally published 1999, with additional stories from 1969-1973). 369 pages.
Delightfully geektastic. Time-travelers visit a past where the lines between fiction and reality are blurred; they go to the Mars of this past, where they meet the creations of Edgar Rice Burroughs, H.G. Wells, C.S. Lewis, Ray Bradbury and others. I loved it, but you probably have to be a big fan of classic science fiction to like it or even follow it.
9/10 (One of my favorites).