This post is actually about two different pairs of books, both by Scott Westerfeld:
The Risen Empire and The Killing of Worlds, really one book in two bindings. (The epilogue from the first book is reused as the prologue for the second book.)
Peeps and The Last Days, which is actually an honest sequel about different characters. (The ones from Peeps only show up after the halfway point.)
After reading these two close together, you might get the impression that Scott Westerfeld enjoys the ways large-scale biological systems work. The Risen Empire (hereafter used to refer to the entire story) contains repeated references to cats and how they allowed the human race to evolve at various stages. Peeps contains repeated references to parasites and how they allowed the human race to... well, okay, that isn't quite true. A lot of the parasites in the book are (a) pretty nasty to their hosts and (b) not very interested in humans. When every other chapter except one talks about a different type of parasite, though, you could say biology is a pertinent subject.
So what are these books actually about?
The Risen Empire is an enjoyable space opera set in the eponymous space empire, which is ruled by a god-king, the Risen Emperor. You see, the twist to immortality is that you have to die before the (presumably artificial, but it isn't completely explained) symbiont which provides life can bond with your body. The problem with immortality is that it is used to reward the emperor's loyal servants (typical lifespan without the symbiont and with good medical care: 200 years) and they are slowly accumulating all the wealth in the empire... and are not interested in change. As a result, other groups of exiled humanity not under the auspices of the Empire are advancing rapidly technologically, while the Empire falls behind. The Rix cult is the group in question here.
I enjoyed this book a lot. It has a classic feel (there's nanotech, but it can't do everything, unlike the nano in The Golden Age) and still manages to have an interesting plot and lots of surprises. (There is a good one in the first chapter.) For most of the book, you know about the existence of the Emperor's Secret, something which could bring down the Empire, but not what the secret is. (At least, I didn't quite guess it before it was revealed.) Sadly, there are several potential plot threads left dangling for sequels to pick up, and Westerfeld's FAQ states that he'll maybe write them someday, when he's rich enough and secure enough to not need or want the money and attention he gets from writing young adult books. Oh well.
Onto Peeps: this is a vampire story, although it isn't obvious in the first chapter. The story: vampirism is caused by a parasite that infects humans, and Cal Thompson is one of a few rare carriers who are genetically immune (at least partially) to the effects of the disease. But the girls he kissed before he found out he had it aren't...
If you're bothered by parasites and rats and other gross things, don't read this. Also, it's labeled young adult (The Risen Empire is somewhat adult), but the parasite encourages behaviors which lead to it spreading: biting, scratching, kissing, and all that that implies. The Last Days is an honest sequel that starts off on a different path with some teens forming a band. Is it a problem that their singer has the disease and the world seems to be ending, err, sorry, there are just some waste disposal problems, we'll have them figured out in a few months? The tone is very different from biology major Cal's clinical narration, which I think is a plus.
I enjoyed Peeps a little more than The Last Days, but you might as well read them both together. Also, I hated the plot twist near the end of Peeps. It's one of those things I should have seen coming.
It's important to know where to end: The Risen Empire does a better job of that than Peeps or The Last Days, I think. And now I'll do the same.