You press your physics textbook flat against a vertical wall with your hand, which applies a force perpendicular to the book. what is the direction of the friction force on the book due to the wall?

Friction or Frictional force will always be acting opposite of the applied force. If you apply a force perpendicular to the book, assuming that it does not move, then it will still possess a static friction, unless it move in any direction up or down, then the direction of friction will always be opposite on the applied force.