added filer/maping funciton to bookshelf to sort books by category (currently/want to/read)

src/Bookshelf.js

@@ -24,12 +24,12 @@ class Bookshelf extends Component {
 
                   {books.filter(function(book){
                     return book.shelf === "currentlyReading"
-                    }).map((book) => {
-                    return
-                      <li>
+                  }).map((book) => (
+
+                      <li key={book.id}>
                         <div className="book">
                           <div className="book-top">
-                            <div className="book-cover" style={{ width: 128, height: 193, backgroundImage: 'url({book.imageLinks.thumbnail}")' }}></div>
+                            <div className="book-cover" style={{ width: 128, height: 193, backgroundImage: 'url("{book.imageLinks.thumbnail}")' }}></div>
                             <div className="book-shelf-changer">
                               <select>
                                 <option value="none" disabled>Move to...</option>
@@ -44,7 +44,7 @@ class Bookshelf extends Component {
                           <div className="book-authors">{book.authors}</div>
                         </div>
                       </li>
-                  })}
+                  ))}
 
                 </ol>
               </div>