Jack Campbell - The Lost Fleet series. These are dumb but fun. A space captain gets frozen at the start of a war, and is thawed out after a century of conflict. He teaches the new generation the value of obeying orders, respecting the laws of war, and so on. About a dozen paperbacks.
Jared Diamond - Guns, Germs and Steel - trade paperback. Civilizations developed the way they did due to their environment.
Jared Diamond - Collapse. Hardcover. Civilizations collapsed because they did not respond well to changes in their environment.
Jared Diamond - The World Until Yesterday. Hardcover. Civilizations did things differently in the past, and that helped keep them from collapsing.
Nobuhiro Watsuki - Rurouni Kenshin - three huge trade paperback manga. A wandering pacifist samurai in Meji-era Japan.
Robert L. Forward - Rocheworld (4 books in series). Solar-sail powered expedition to another star system. They discover a binary planetary system, with two planets orbiting each other so closely that they share an atmosphere.
Robert L. Forward - Dragon's Egg (2 books in series). Intelligent, flat beings live in the high gravitational field of a star.
Stephen Gould - Jumper series - hardcover and trade paperback. Excellent series about someone who can teleport.