Why does a phone book not leave bruise when using it with punch?

A phone book does not leave a bruise when used with a punch because it is not heavy enough to cause blunt force trauma. Bruising occurs when the impact of a heavy object causes blood vessels under the skin to burst and leak blood, resulting in a discoloured area of skin. While a phone book may be able to cause some minor pain or discomfort when used to strike someone, it is unlikely to cause serious injury.