There are a few students who enjoy the Stevenson book, but most of them want something a little more modern. After the 7th grade reads The Face on the Milk Carton, I always get requests for similar books. I will just list most of them, since we pretty much know the plots! Notable exception: Lee Weatherly's Missing Abby (2004), which I read last night. Although this is a tremendously sad book, it was compelling. Emma runs into her former best friend Abby on the bus. They have words, and after that Abby goes missing. While searching for Abby, Emma has to come to terms with how their friendship fell apart, and how she misses not only Abby, but the way she used to be with her. Good mystery, but also evocative of that great middle school Trauma Extraordinaire-- losing former best friend. You can guess the ending if you pay close enough attention, but it still is a gut-punch. Highly recommended.
Others to consider: Martin's Missing Since Monday(1986), Kehret's Abduction(2004) and Deadly Stranger(1986), Mazer's Solid Gold Kid(1977) and Taking Terri Muller (1981), Griffin's Vikki Vanishes (1995), and Duffy's Missing (1988), which is oddly enough a book that is lost with alarming frequency!