Who did Jem send a note to and what did it say?It's in Chapter 5.

In Chapter 7, Jem and Scout write another note intended for Boo.

   "Dear sir," said Jem. "We appreciate the--no, we appreciate everything which you have put into the tree for us. Yours very truly, Jeremy Atticus Finch."
   "He won't know who you are if you sign it like that, Jem."
   Jem erased his name and wrote, "Jem Finch." I signed, "Jean Louise Finch (Scout)," beneath it. Jem put the note in an envelope.

But the next morning, a sad surprise awaited Jem. The secret knothole had been filled with cement. There would be no more conveying of messages with their unknown friend.