Closing the Old Man's War Scalzi goes out on a very smart and once again fun and superlative story. The book is full of surprises and it's best to go into it without knowing much. Just know that it involves the usual high stakes and adds a lot of layers of deception and tricks from John Perry and Jane Sagan.
It is a worthy conclusion for the trilogy and once again makes us wonder if humanity wouldn't be its own worst enemy, even when at war with alien species from all over the universe.