The Woman in the Window by A.J. Finn

Anna's lives near busy New York City but her only connection with the outside world is through an online chat room for agoraphobics.  She fills her days with her favorite movie genre - film noir - and phone calls to ???.  Oh yes - were Hitchcock alive, he would be making the movie.  Then a family moves in across the street.  They seem friendly - particularly the teenage son.  One day, watching from her window, she sees something terrifying through theirs.  Or does she?  She tries to convince the police of what she saw but things just don't add up.  Reality starts to mix with all those movie plots until it is hard to tell one from the other.  As in all these novels of building psychological terror, it is always fun to discuss just when did you as the reader figure it out.