What memoirs would Jem have had after the night of finding gifts in the tree and meeting with Tom Robinson's family and Robert Ewell?

Jem is so surprised and grateful to find trinkets in the tree that he actually writes a letter of thanks to the giver of those gifts. Jem is heartbroken when he discovers that Mr. Radley has filled up the hole in the tree with cement. At the end of this chapter (7), Scout can hear Jem crying. This has clearly affected Jem. We cannot be sure that he knows that Boo Radley had been giving the gifts. However, it is possible. In any case, had Jem written a memoir about this event, he probably would have expressed how excited he was to find new surprises in the tree. It is somewhat magical, like finding presents under the Christmas tree. And if he knew this was Boo's way of reaching out to someone, he would have written about it as a turning point in how he understood Arthur Radley.

In Chapter 26, Jem and Dill go with Atticus to Tom's house. Atticus breaks the news to Helen that Tom has been killed. They see Helen collapse in grief. Of the three children, Jem was the one most disappointed when Tom was convicted. Seeing Helen's grief probably added to that disappointment and frustration. Had he written a memoir about this event, especially as an older, wiser man, he would probably express his dissatisfaction with the racist element in Maycomb and how it led to a wrongful conviction and a family losing a husband and a father.