The Ghost Brigades (Old Man's War, #2) by John Scalz

The military SF sequel to his extraordinary Old Man's War, John Scalzi's The Ghost Brigades is the second in The Old Man's War series.

Who can you trust, if you can't trust yourself?

Three hostile alien races have united against humanity, determined to halt our expansion into space. The mastermind behind this lethal alliance is a traitor – Charles Boutin. He was a Colonial Defence Force scientist, with access to their biggest military secrets. Now the CDF's only hope is to discover Boutin's plan. Trouble is, Boutin's dead.