Old Man's War (Old Man's War, #1) by John Scalz

Perfect for an entry-level sci-fi reader and the ideal addition to a veteran fan’s collection, John Scalzi's Old Man’s War will take audiences on a heart-stopping adventure into the far corners of the universe.

John Perry did two things on his 75th birthday. First he visited his wife’s grave. Then he joined the army.

The good news is that humanity finally made it to the stars. The bad news is that, out there, planets fit to live on are scarce―and alien races willing to fight us for them are common. So: we fight. Far from Earth, the war has been going on for decades: brutal, bloody, unyielding.