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.
Jem and Dill get the idea to write a note to Boo Radley asking him to come outside and tell them what he does inside his house. Furthermore, they write that they will buy him an ice cream and will not hurt him. They put the note on the end of a fishing pole and try to attach it to a window shutter on the side of Boo's house.
"We're askin' him real politely to come out sometimes,
and tell us what he does in there- we said we wouldn't hurt him and
we'd buy him an ice cream."
Jem wrote a note to whoever was leaving all the nice things which was actually Boo Radley. Boo Radley wanted to communicate with the children and be friends with them, but was later cut off because of his brother who did not want any more interactions and plugged up the tree with cement by saying it was dying.
Jem wrote a note to the person leaving stuff in the knothole which was Boo Radley to thank him for the gray ball of twine, the soap carvings, the gum, the spelling bee medal, and the pocket watch.
Jem wrote a not to whoever was leaving them thing in the knothole it just thanked them for what that person was doing for them