A girl press her physics text book against a rough vertical wall with her hand. The direction of the frictional force on the book exerted by the wall is

Friction forces are always parallel to the surfaces in contact, which in this case, are the wall and the cover of the book. This tells us that the friction force is either upwards or downwards. Because the tendency of the book is to fall due to gravity, the friction force must be in the upwards direction.