Novik is particularly good at putting her characters in godawful situations, extracting them, and putting them back in godawful situations. And the beauty is that she doesn't sacrifice her characters for this. (She does kill off characters, but doesn't have them act out of character. So I feel this distinction needed to be made.)
That being said, I didn't think things could get worse than all the dragons getting sick. Not only sick, fatally ill. When Temeraire mistakenly flies into a quarantine zone, he proves that he has immunity - and everyone is desperate to figure out why. Most of this book takes place with Temeraire, Laurence, their crew and assorted other dragons trying to figure out why Temeraire isn't getting sick - and how to immunize the rest of the dragons.
The last part of this book reveals a shocking battle tactic, and one that Temeraire is morally opposed to. It also is inevitable, given who Laurence and Temeraire are - and puts them in worse spot than they were before.
I found this absolutely thrilling, but also had a hard time getting through this book because a lot of it - the parts dealing with the sickness - were too depressing.