Or more specifically, Jon's wish for "something interesting to happen" caused his mother to come into possession of that occult spell book, which caused him to switch bodies with his sister.
Perhaps he couldn't wish himself back to normal. But maybe there was a spell in that book that could reverse what was done to him. Since the use of the book would be considered "something interesting" (as far as the stone was concerned), maybe he could use it to fix his problem.
But first he had to get the book. And he didn't think the stone would let him just wish for it. He had to go find it himself.
Little did he know, though, his sister had the same idea.
