"The Old City" is, in my opinion, the best kind of sequel.
In her debut novel Katherine Sorin set up a believable world and an admirable protagonist, and in her next one both of them grow in some unexpected ways. It isn't just the same story told again (as so many sequels, unfortunately, tend to be) it is the next phase of an ongoing story.
I really don't want to say much about what happens in "The Old City" because it is just as tightly plotted as "City Of Whispers" and to reveal anything is to reveal too much. Suffice to say that it picks up where we left off, same characters, same world, but... different.
The writing style is subtly different, making use of dramatic flashbacks that help to explain both the back story of the main character Ailis and foreshadow things to come without slowing the pace of the action. It's clear that Sorin is stretching herself as a writer, and I suspect she hasn't hit her stride yet--I am really looking forward to more stories from her.
Ailis continues to grow and change as a person, coming to terms with her experience and being changed by them, without losing the intimate voice that made "City Of Whispers" so enjoyable. The world also expands--nothing that was explained in the first book is contradicted in the second, but much is added.
If you have read "City Of Whispers" then you will be pleased and excited and surprised by "The Old City". If you haven't read "City Of Whispers", then read it first, then come back and get this one.