The first book (the one pictured) is ok, trying a bit too hard to be Starship Troopers, but the next two books in the series are pretty good.
The premise is that Earth is being artifically kept in a perpetual state of
Next Sunday, A.D. by the Colonial Union (the government of the humans who live in space/on colonies). The Colonial Union is at war with every single other species in traveling distance (and there's upwards of 4-500 of them), and in order to fuel this perpetual struggle for territory, they keep Earth largely in the dark about the state of the galaxy at large, and the only way to get off planet is to enlist in the Colonial Defense Forces. You sign a letter of intent at 65, and are recruited at 75. After that, you go up to the space station and nobody ever sees you again.
What really happens from there is, due to espionage and conquest, the CU is way, way ahead on technology than what earth is. They have the ability to regrow you a new body from your own DNA, but alter it to superhuman standards complete with intracranial computer (BrainPal!(tm)), Captain America-like strength and reflexes, artifical SmartBlood(tm) which is immune to disease and many times more effective than regular blood both at oxygenation, clotting, etc... and they can then transfer your consciousness from your old, broken down body into this new permanently 22-years-old superbody to fight the screeching, chittering hordes for interstellar real estate. Then, in 10 years you can retire and they'll put you back in a 20 year old regular human body (also grown from your own DNA), and let you become a civilian colonist. Or you can re-up.