As you might expect, I’m searching for a book.  I probably read it in the 1978 to 1980 time frame, so publishing occurred before that.  It is possible I read the book as late as 1982, but I don’t think so.

The book may have been a Scholastic publication, but they didn’t find anything matching the rest of the description.

The book was a short story collection.  I do not remember the title or author or authors or editor.  I think the version I read was a trade paperback, borrowed from the school library.  During 1978 to 1980 I was in 5th and 6th grade.  I suppose I could write to the librarians in those schools and ask if they still have books matching the descriptions.

The story I remember from the book had a boy in probably 5th or 6th grade walking alone in the woods one evening.  A UFO appeared through the trees (maybe he was in a clearing?) and shone a blue light through the center of his chest.  Not long afterwards a third arm started growing where the light struck the boy.  He was teased at school for being different, and he thought of having the arm surgically removed.  But the arm proved to be useful when the bullies in his school started a fight with him.  He elected to keep it.  He finished the story by saying other people around the world were getting struck by the same blue light and were growing arms as well.

I do not remember the remainder of the stories in the book.  I’m not sure I actually read them; I may have had to return the book to the library.