I Hate Your Guts, Ben Brooster

When eleven-year-old Charlie's nine-year-old cousin Ben arrives from Japan with the wrong suitcase to spend a year with him, Charlie is irritated by Ben's genius intelligence, but comes to admire him as the two are drawn into a mystery concerning a handwritten will in Ben's suitcase.Charlie is excited about meeting his cousin, Ben, the genius, but soon Ben is doing some pretty stupid things, getting Charlie into a mess of trouble