The portrayal of Nicholas's genius especially interests me, and when we get inside his mind and follow his observation and reasoning, and when he takes specific action such as figuring out how to make a key mold and then a key on the sly, his intellect is most convincing. I also love that his problem-solving includes clever ruses and acting at times, as well as puzzling things out in the expected mental-exercise sort of way. Other traits, such as his photographic memory and especially his extreme ability to speed-read, seem a little more like plot conveniences. We have to take the author's word for these more. On the whole, Nicholas is likable and believable, and part of the charm of the book is that though several of the characters seem stock (Mr. Collum, the bully group known as the Spiders, Nicholas's friend John), we don't care, because they come alive. In fact, that these are familiar characters coming alive just adds more enjoyment to the book.
Another thing I adore about this book is that it's one to sink down into. It's thick (468 pp.), it's definitely upper-MG reading level even though the MC is only nine, the writing is descriptive and all-around stellar, and Stewart has the confidence to take his time getting the mystery plot rolling -- not something a mystery writer normally does. As far as the reading experience goes, people who like Harry Potter or older-style MG that's a little longer, a little slower-moving, and a little higher reading level than a lot of fare today, will enjoy this. As will readers of smart mysteries such as Blue Balliett's or Elise Broach's books. Highly recommended.