I am assuming that you are asking what Boo would have written about regarding the happenings in To Kill a Mockingbird. Given that readers are not ever given a clear picture of Boo, they can only characterize through indirect characterization, much could be learned through Boo's diary.
Possible diary suggestions are as follows:
-Boo's reasoning behind leaving the items in the tree.
-Boo's thoughts on Jem's dare to touch his house.
-Boo's thoughts on the night he saved the children.
-Boo's thoughts on why his father keeps him locked up in the house.
-Boo's thoughts on his past (why his father chose to keep him locked up in the first place).
-Boo's thoughts on Atticus shooting the dog (mentioning his thoughts on Atticus as a father versus his own father).