Although I found Night Watchman Express to be an enjoyable and fairly easy read, I did have a couple of issues with it.
First, there is just too much story and information here for one book. The book is broken into two distinct parts, which I personally feel would have been better as two separate books. The storyline in the mansion could have been expanded on greatly and I also would have liked to see the island life described in greater detail.
Second, although labelled as a steampunk novel, to me, steampunk elements were only a small portion of the book. Instead it seemed to be much more about island magic. This was a little disappointing as I am a big fan of steampunk.
I do think that Night Watchman Express is a book that middle grade readers, especially would find entertaining. The character or Riki in particular is fun and wild and Miriam and Mana are just two of the various strong female characters in this book.