FLAT STANLEY : INVISIBLE STANLEY - Harper Collins

"Disappearing act
One morning, after a terrible storm, Stanley Lambchop is nowhere to be found. His family can hear him, and there is a lump under his covers, but no one can find him! Just where is that boy?
Then they discover the truth -- Stanley is invisible! At first, Stanley is very busy. There's so much for an invisible boy to do. But will he stay that way forever?"