text: // REDUCER
  // All reducers get two arguments: state & action
  // State argument is not application state, only the state this
  // reducer is responsible for
  // state = null --> default state argument to null
  // if the arugment comes in undefined
  export default function(state = null, action) {
    // If action.type is BOOK_SELECTED, then return the payload
    switch(action.type) {
      case 'BOOK_SELECTED':
        return action.payload;
    }
    // If the action was anything else other than BOOK_SELECTED,
    // return state as null and move onto the next reducer.
    return state
  }
  