Most of the books I read that are not fantasy-based are history books. I keep going back to William Urban's The Teutonic Knights and Medieval Mercenaries for inspiration, and I love Peter Marren's Battles of the Dark Ages. Military Blunders by Major Steven Eden, U.S. Army (Hey that's how he's credited on the cover so that's how I cite him) is a great piece for armchair historians and it helped me with some lessons on occasion.
The only non-fantasy author I actively seek out is Jodi Picoult. I've read all of her books since I was given The Pact in high school, and although they are all the same in terms of structure, there's something about her writing style and the characters that makes me come back to them. It also helps that I attended a luncheon with her as the guest speaker, and her staunch position on researching a subject to know it inside and out really stuck with me. If anyone would like to get into her work, I strongly recommend either The Pact or My Sister's Keeper.
