I always get my dad books because he never makes a wish list, but I know he reads a lot.
I try to get him books I think he'll like, but not necessarily ones he would pick up on his own. This year I got:
The Fire: The Bombing of Germany 1940-1945 -- It's translated from German and details the Allied bombing campaign on Germany during WWII from the German perspective. My grandfather was a bombardier during WWII, so he likely was involved in some of the campaigns detailed inside.
Summer for the Gods: The Scopes Trial and America's Continuing Debate over Science and Religion