When You Reach Me by Rebecca Stead

[Pretend to find letter in book.  Read letter.]

Things in Miranda's life has been a little topsy turvy lately.  Her best friend Sal gets punched in the face by another kid for no reason.  A crazy man shows up on the corner of her block.  And the apartment key Miranda's mom hid has been stolen.  Then this note shows up.  What does it mean?  Who is M? Why does this guy need a letter? And most importantly, what is Miranda going to do about it?