The Immortals: The Complete Book. Symphony of War

Broken and crippled from an interstellar war, Nicholas Caddy is augmented and specially trained to pilot the Immortal Armour, an engine of war arriving just in time to face down a threat much more devastating than anything the Colonized Worlds could ever conceive of. A prequel to the novel Symphony of War and featuring many of the same characters, this novel-length collection features four stories of Caddy's war, with a bonus short story at the end.
Set over a decade before the events of Symphony of War: The Polema Campaign.