index.js

export const update = (book, shelf) =>
  fetch(`${api}/books/${book.id}`, {
    method: 'PUT',
    headers: {
      ...headers,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({ shelf })
  }).then(res => res.json())

updateState = (res) => {
    console.log(res); // defined
    this.setState({currentlyReadingBooks: res.currentlyReadingBooks, // undefined
      wantToReadBooks : res.wantToReadBooks, 
      readBooks: res.readBooks});
  }

  updateBookShelf = (book, shelf) => {
    const promise = BooksAPI.update(book, shelf);
    promise.then((data) => 
    this.updateState(data));
  }