I know my daughter loves these but they make me crazy. The Alden children are wholesome, sweet and industrious - gag-inducingly so. Their grandfather is a kind rich indulgent "semi-parent" but often the children (the oldest is either 14 or 15) are left entirely to their own devices. They are very much "those meddling kids" in all the "mysteries." Like many of the books that were written after the original author passed away, this one is awfully paced. It drags out the exposition for five or six chapters, builds very little suspense and then wraps everything far too neatly in the last chapter. In this one particularly there are too many characters acting mysterious at the same time and not enough detail is given to make any of them believable. Now to the positive: they are simple reads and my 8 year old loves them, this one included.
What do You think about Disappearing Staircase Mystery (2001)?