The Mystery at Lilac Inn (Nancy Drew, Book 4) by Carolyn Keene

Nancy and her friend Helen visit their friend Emily Willouby at the Lilac Inn, which Emily now owns, to help her plan her wedding. Emily plans on selling inherited diamonds in order to help fix up the Lilac Inn. However, Nancy soon learns that someone has been impersonating her and making expensive purchases under her name. Soon after, Emily's diamonds are stolen! Can Nancy find the thieves and recover the missing diamonds?