This was a quick and fun continuation into the SPECTR world that Ms. Hawk started building in the first book. Caleb's still possessed - obviously - and he and Gray are learning how to share one body and work in tandem to fight the monsters. The complications of the situation aren't ignored either - Caleb still very much wants to be rid of Gray, and there are still questions being asked on all sides about the ethics of "monster" killing, at least where Gray is concerned.
The main plot with the titled Master of Ghouls felt a little thin on substance but there's hints of something else in the works for the rest of the series, so I'll wait to see where that goes.
Pointing out it's only been eight days since all this started makes the insta-love a little harder to swallow, and Jon bordering on unprofessionalism due to Caleb feels like a stretch.