I picked this up in a bargain bin cause I liked the cover art.
But - I was surprised TOR published this. This was really not a good book.
After an unsuccessful mission to colonize a far planet, a generational spaceship has aborted that mission and decided to return to earth - and conquer it.
The elite of the ship have viciously put down a mutiny against this, imprisoning a great many 'mutineers' to the lower levels, and considering them subhuman. They also believe that the inhabitants of earth have, post-apocalypse, degenerated to a subhuman level. (Neither are actually subhuman.)
Both ship and earth have mysteriously and unexplained-ly suffered diseases which cause most people past puberty to die.
Not only is this really major coincidence never dealt with, characters just don't behave as if they are in the situations that the author puts them in, or have had the life experiences referred to. There's no character believability. There are references to our era that generations of teenagers would have long forgotten - the people in the book don't really seem to be people of their world. For example, they keep referring to each other as 'kids,' expecting a certain level of immaturity - but if people consistently die in their 20s or younger, they would fully be considered to be adults, and to behave as such. It was just stupid.
And the story didn't really go anywhere.