I read mostly fiction as it is my escape from a long hard day (I read before going to bed at night) But when I do read non-fiction it is generally military history stuff (mostly U.S. and German WWII stuff)
As for fiction, I run a gamit I guess. I love sci-fi and fantasy, everything from Star Trek novels, Tolkien, Alistar Reynolds, CS Lewis (both Narnia and Space Trilogy), D&D stuff, to just about any cool looking/sounding sci-fi or fantasy I can find. Last time I was deployed I had an old book from my spouse's aunt, Lawrence Sander's The Tomorrow File. A little strange to get into a first, but then I couldn't put it down. Excellent book that shows what the future could very well turn out to be like.
I also like mystery and horror novels. I'm currently reading a "historical fiction" novel about Vlad Tepes and its pretty neat. Will move on to Agatha Christie's Death on the Nile next (my Grandma sent it to me in a care package here in Afghanistan).