Now that I know that this is a novelization of a play (adapted from a novel), things make a bit more sense, but this still isn't a fantastic book.  There were a few elements I liked, like the heroine as an older lady, and the master criminal, the Bat, was just a regular guy.  Knowing this is based on a play, sort of, explains why characters frequently come and go, why some rooms are never seen, and the short time frame in which the action takes place.  However, since I was reading a novel, I'll note: characters behave unrealistically (someone has been shot and you just wander off by yourself in the dark?), some villainy being revealed early spoiled some of the mystery aspect, and the servants' behavior (which I guess would be "humor" on stage).  There is a LOT of classicism with the attitude toward the servants, and (CW) some casual racism towards the Japanese butler (who is referred to as "the J--" a few times).  I understand that there were significant changes between the original novel (The Circular Staircase) and the play, but it still feels like maybe The Bat as a novel isn't really necessary.