Chronoliths

One day in Thailand, 21st-century slacker Scott Warden witnesses an impossible event: the violent appearance of a 200-foot stone pillar. Its arrival collapses trees for a quarter mile around its base. It appears to be composed of an exotic form of matter. And the inscription chiseled into it commemorates a military victory...sixteen years hence.
As more pillars arrive all over the world, all apparently from our own near future, a strange loop of causality keeps drawing Scott into the central mystery--and a final battle with the future.