code: public synchronized EvernoteLinkedNotebookHelper getLinkedNotebookHelper(@NonNull LinkedNotebook linkedNotebook)
              throws EDAMUserException, EDAMSystemException, EDAMNotFoundException, TException {
  
          String key = linkedNotebook.getGuid();
          EvernoteLinkedNotebookHelper notebookHelper = mLinkedNotebookHelpers.get(key);
          if (notebookHelper == null) {
              notebookHelper = createLinkedNotebookHelper(linkedNotebook);
              mLinkedNotebookHelpers.put(key, notebookHelper);
          }
  
          return notebookHelper;
      }
docs: Returns an async wrapper providing several helper methods for this {@link LinkedNotebook}. With
  {@link EvernoteLinkedNotebookHelper#getClient()} you can get access to the underlying {@link EvernoteNoteStoreClient},
  which references the {@link LinkedNotebook}'s note store URL.
  
  @param linkedNotebook The referenced {@link LinkedNotebook}. Its GUID and share key must not be
  {@code null}.
  @return An async wrapper providing several helper methods.