This third volume is mainly concerned with Bitterblue (of the title), now Queen of Monsea, 18 years of age, trying to make sense of her role as ruler and picking up the pieces after the demise of her father's wicked reign. All of the main characters from Grace appear and play an important part and contribute to the extensive plot. Bitterblue, quite rightly, does not know who to trust and meets some other interesting characters in the city.
Quite complicated and often less-than-uplifting plotting leads to a reasonably happy conclusion and an optimistic future. It's a great read although I would advise reading the first two volumes beforehand (Grace, Fire). Aimed at the young adult reader, all lovers of fantasy should enjoy it.