The Mystery at Lilac Inn

The Mystery at Lilac Inn by Carolyn Keene is 182 pages.  It is a Nancy Drew book.  Nancy goes to her friend Emily's hotel.  Emily inherits diamonds from her mother, but they get stolen.  While looking for the diamonds, Nancy discovers a secret passage.  At the same time, Nancy finds out that she has a person impersonating her and stealing some of her things.  The housekeeper at the hotel starts to act strange.  Nancy witnesses her sneak away.  This makes her suspicious of the housekeeper.