Brief Synopsis: The novel is set in an undefined distant future (although it is implied to exist roughly 5000 years from now), in which there is a galactic empire spanning eighty worlds, amongst other human civilizations. Empire sends war hero Laurent Zai to rescue the Child Empress, taken hostage by the nefarious and powerful Rix, a group of machine-augmented humans bent on seeing an AI hive-mind as ruler of the Eighty Worlds. The storyline continues in The Killing of Worlds. In this page turner, the Emperor has a terrible secret, a secret Nara is in danger of finding out. Even though this is said to be the second half of the story, the ending leads one to believe that there may be a third book waiting to be revealed. 
Click here for a conversation with Westerfeld about The Risen Empire.
“The Risen Empire and Killing of Worlds were written as one 190,000-word manuscript, though I hadn't expected it to be that long. Borders or somebody came out with an edict that they weren't going to support anything with a price point of over $25, and since then a whole bunch of books have been famously split up. From the submission draft to Tor splitting it in two, I did a lot of structural refiguring. There were a lot of changes, and I think the book is actually better because of them.”
-Scott Westerfeld