A Fire Upon the Deep (Vernor Vinge)

This is one of my very favorite science fiction books of all time. Vinge has created an amazing, mind-blowing universe, filled it with fascinating aliens, set up a truly epic battle between good and evil, and laid it all on the shoulders of characters, human and non-human, you really come to care about. There is action ranging from galaxy-wide cataclysms to up-close-and-personal betrayals. There is technology ranging from the literally unimaginably godlike to muzzle-loading cannon and wheeled carts. Highly recommended.