I need help with the name of a book(s) I read as a kid. It was part of a series of short stories.?

There was this big tree with a tree house in the school playground. The only way up was a long rope with knots in it. The tree house was so high up no one can make it all the way. A boy went to the tree house after hours and tried to climb it, he got midway and stopped because he saw his ghost with a broken neck.

There was also another story, it may have been a different book. A man was walking down the street and came upon a little boy by himself. The boy was cold so the guy loan him his green stripe sweater and walked him home. The next day the man goes to the kids home to get his sweater back. The kid's mom answers the door and tells him her son is dead, has been for years. The man goes to the grave site and find his sweater laying on the undisturbed ground where the boy is buried.