My personal favorites (I've read all but the last one):
1. A Farewell to Arms (Ernest Hemingway): A semi-autobiographical novel, this is guaranteed to make any person cry, one of the best works of Hemingway. Very beautifully written in telling sentences.
2. Slaughterhouse-5 (Kurt Vonnegut): A satire which mixes elements of science fiction with the bombing of Dresden in 1945. The author was there at the time, as a Prisoner of War. He actually survived the fire-storm by sheltering in a slaughterhouse himself, and hence the name.
3. The Face of Battle (John Keegan): This book dissects an ordinary soldier's experience in 3 battles - Agincourt, Waterloo and Somme. An absolute classic of its times.
4. Longitude (Dava Sobel): Describes 18th century British clock-maker John Harrison's invention of a timepiece accurate enough to measure longitude at sea.
5. Dispatches (Michael Herr): Haven't read this one, but have read rave reviews of the same.