A recommendation from MP, this is apparently one of the classics of the genre of Military Science Fiction. In addition to being a fun and quick read, I found it thought-provoking, and a really cool concept.

In a near-future earth, humans have traveled to other planets and made contact with aliens. As it turns out, our galaxy is teeming with intelligent, multi-planetary species far more advanced than us. And they're aggressive! To protect earth and its colonies against this thread, the humans have created the Colonial Defense Force (CDF). This galactic army is shrouded in mystery to the earth dwelling humans, which are bound to the planet. The only way for a human to leave earth is to take a leap of faith and sign your future septuagenarian self to fight for the CDF. And you can never go home again.

The book is not really profound, but skims some deeper than expected themes and references, including Arendt's banality of evil. This surpassed my expectations given the genre.

• Reincarnation: what happens when an army division consists entirely of octogenarians?
• Nature vs nurture (ghost brigades): what would it be like to be a completely different person with roughly the same set of genes?
• What is it like to love a person deeply, lose them, and then meet their clone?
• What is it like to gain a completely new body after living a full and long life?
• What would it be like to be battling a huge variety of unknown species all the time, where you have absolutely no idea what to expect?

Choice quotes:

The problem with aging is not that it’s one damn thing after another—it’s every damn thing, all at once, all the time.

Fun tech mockery, like naming a personal assistant "asshole" and the resulting potty humor including “Activate Asshole” and "Go away Asshole", as well as the assistant confirming that "I am Asshole". And other crass hilarity like a female soldier activating her assistant with "Hey Bitch".

I enjoyed the world building quite a bit, from the whole premise of the book, to the various alien species encountered:

• The Consu, vaguely culturally Japanese, are a very advanced and enlightened species capable of destroying the humans easily, but interested in more profound goals. Honor and tradition seems to play a huge role in their culture, as is ritual honor based suicide clearly modeled after harakiri.
• The Whaidians, vaguely culturally Arabic, are brutally destroyed by the CDF. I suspect there is a political agenda here as well, with parallels to the War on Terror.
• A variety of other species which were only alluded to. A terrifying slime-mold which enters through your mouth and eats you from within. The Covandu, a tiny but intelligent species with high-tech weaponry, but so small they can be crushed underfoot like a bug.

Open questions:

• Why don't animals photosynthesize in addition to regular metabolism?