The Empire of Gut and Bone (The Norumbegan Quartet Series #3)

The land of New Norumbega is an unusual one - an empire of gut and bone, lost within the organs of a huge, alien body. At its dark, dry heart is a ruling class that doesn't care about much besides itself, especially its mechanical servants, who are tired of being taken for granted.
Brian and Gregory have come to New Norumbega for a reason - to get the Norumbegans to help them thwart an alien attack back home on earth. But instead, the two boys find themselves caught up in both a robot rebellion and a murder mystery after one of the Norumbegan leaders is sent to sleep . . . permanently.
In New Norumbega, it's very hard to know who to trust. There are assassins around every corner, and secrets pave every conversation. Brian and Gregory will be lucky to make it out alive, never sure if they are meant to catch the murderer . . . or be the killer's next victim.