Split the dataset into a train and test set with the [~datasets.Dataset.train_test_split] method:

books = books["train"].train_test_split(test_size=0.2)

Then take a look at an example:

books["train"][0]
{'id': '90560',
 'translation': {'en': 'But this lofty plateau measured only a few fathoms, and soon we reentered Our Element.',
  'fr': 'Mais ce plateau élevé ne mesurait que quelques toises, et bientôt nous fûmes rentrés dans notre élément.'}}