There is a reference to the seventh book during the time at Grimauld Place during Christmas. When you are helping Ginny find Kreacher's items and you pull off the painting from the wall, a locket can be seen hanging from the wall. Ginny says it will not open. This is one of the Horecrux that is mentioned in the sixth and seventh books--R.A.B.'s Locket, which he stole from Voldemort.