"Excellent"
True, the plotting is a little hamfisted, but it's not too bad. Besides, let's face it: the plot is merely a convenience for the author as he peels layer after layer from his ingenious onion of a cypher. A truly intelligent book in a time when intelligence in writing is rare. It's witty and clever, and I was always racing to solve the mysteries ahead of its characters.
Regarding the narrator, at first, I didn't like him, but as he demonstrated his facilities with voices and accents, I began to realize how exceptional he is. I recommend his interpretation highly!