text: //all reducers get 2 arguments: the current state, and action
  //reducers are only called when an action occurs
  //the state argument is not the application state, it is the state this reducer is responsible for
  export default function(state = null, action) { //ES6 syntax. if state is undefined, set it to null
  	switch(action.type) {
  	case 'BOOK_SELECTED':
  		return action.payload;
  	}
  
  	return state;
  }