text: // The state argument is not related to the application state,
  // it reffers only to the reducer state and it goes and returns
  // everytime an action occurs and the reducer is called.
  
  // Action is an object like { type: 'BOOK_SELECTED', payload: { title: 'React and Redux'} }
  export default function(state=null, action){
  
    switch(action.type) {
      case 'BOOK_SELECTED':
        return action.book;
    }
  
    return state;
  }
  