Tear Jerker / The Ghost Next Door

• The ending (both the book and the TV episode). The protagonist, after discovering that she was in fact the ghost that the title is referring to and has been dead all along, saves another kid from dying in a fire like she did. Then she rejoins her family in the afterlife.
• The scene right after Hannah discovers what happened to her is particularly gutwrenching. Now that the illusion has been broken, her family is gone, and her house is empty. Desperate for help, she picks the phone up... and immediately realizes there's no one she can call — she's dead. It doesn't matter because the phone is dead, too. She finally just lays down and cries.