a cover is not the book - A cover is not the book means the outside is pretty but the

A cover is not the book means the outside is pretty but the inside is a disaster. Like when you get a pizza and it's just cheese and a sad crust.

That girl looked amazing in the photo. Now she's just a mess in real life.

That car had a cool ad. Now it's just a piece of junk that breaks every 3 days.

That restaurant had a 5-star review. Now I'm eating expired spaghetti.